diff --git a/docs/src/API/System.md b/docs/src/API/System.md
index 379e7fa19b..c7cf3cdd9b 100644
--- a/docs/src/API/System.md
+++ b/docs/src/API/System.md
@@ -7,6 +7,7 @@ optimizations).
 
 ```@docs
 System
+ModelingToolkit.AbstractSystem
 ```
 
 ## Utility constructors
@@ -185,3 +186,41 @@ ModelingToolkit.is_bound
 ```@docs
 debug_system
 ```
+
+## Input validation
+
+The following values can be passed to the `check` keyword of `System` to toggle validation
+of input. Flags can be combined with bitwise `|` and `&`.
+
+```@docs
+ModelingToolkit.CheckAll
+ModelingToolkit.CheckNone
+ModelingToolkit.CheckComponents
+ModelingToolkit.CheckUnits
+```
+
+These can also be used by custom `AbstractSystem` subtypes.
+
+## Utility functions
+
+These utility functions can be useful when manipulating systems, especially when building
+custom `AbstractSystem` subtypes.
+
+```@docs
+ModelingToolkit.collect_scoped_vars!
+ModelingToolkit.collect_var_to_name!
+ModelingToolkit.collect_vars!
+ModelingToolkit.eqtype_supports_collect_vars
+ModelingToolkit.modified_unknowns!
+```
+
+## Namepsace manipulation
+
+ModelingToolkit namespaces variables from subsystems when using them in a parent system to
+disambiguate from identically named variables in other subsystems or the parent system. The
+following functions are useful for manipulating namespacing functionality.
+
+```@docs
+ModelingToolkit.renamespace
+ModelingToolkit.namespace_equations
+```
diff --git a/docs/src/API/model_building.md b/docs/src/API/model_building.md
index cbdf72edae..28069b535e 100644
--- a/docs/src/API/model_building.md
+++ b/docs/src/API/model_building.md
@@ -3,6 +3,29 @@
 This page lists functionality and utilities related to building hierarchical models. It is
 recommended to read the page on the [`System`](@ref System_type) before this.
 
+## Common definitions of `t` and `D`
+
+ModelingToolkit provides common definitions for the independent variable `t` (time) and the
+derivative with respect to it `D`.
+
+```@docs
+ModelingToolkit.t_nounits
+ModelingToolkit.D_nounits
+ModelingToolkit.t
+ModelingToolkit.D
+ModelingToolkit.t_unitful
+ModelingToolkit.D_unitful
+```
+
+Users are recommended to use the appropriate common definition in their models. The required
+definitions can be imported with convenient aliased names. For example:
+
+```julia
+using ModelingToolkit: t_nounits as t, D_nounits as D
+```
+
+Allows using `t` and `D` to refer to `t_nounits` and `D_nounits` respectively.
+
 ## Hierarchical model composition
 
 The `System` data structure can represent a tree-like hierarchy of systems for building models
diff --git a/docs/src/API/variables.md b/docs/src/API/variables.md
index 4c11d900c9..99fd53372c 100644
--- a/docs/src/API/variables.md
+++ b/docs/src/API/variables.md
@@ -18,6 +18,17 @@ Symbolic variables can have metadata attached to them. The defaults and guesses
 at variable construction time are examples of this metadata. ModelingToolkit also defines
 additional types of metadata.
 
+## Variable defaults
+
+Variables can be assigned default values to avoid having to specify defaults to the
+[`System`](@ref) constructor.
+
+```@docs
+ModelingToolkit.hasdefault
+ModelingToolkit.getdefault
+ModelingToolkit.setdefault
+```
+
 ## Variable descriptions
 
 Descriptive strings can be attached to variables using the `[description = "descriptive string"]` syntax:
@@ -56,6 +67,7 @@ help?> u
 ```@docs
 hasdescription
 getdescription
+ModelingToolkit.VariableDescription
 ```
 
 ## Connect
@@ -83,6 +95,7 @@ getconnect(k)
 ```@docs
 hasconnect
 getconnect
+ModelingToolkit.VariableConnectType
 ```
 
 ```@docs; canonical=false
@@ -112,6 +125,8 @@ isinput
 isoutput
 ModelingToolkit.setinput
 ModelingToolkit.setoutput
+ModelingToolkit.VariableInput
+ModelingToolkit.VariableOutput
 ```
 
 ## Bounds
@@ -144,6 +159,7 @@ hasbounds(x[2])
 ```@docs
 hasbounds
 getbounds
+ModelingToolkit.VariableBounds
 ```
 
 ## Guess
@@ -232,6 +248,7 @@ isirreducible(important_value)
 
 ```@docs
 isirreducible
+ModelingToolkit.VariableIrreducible
 ```
 
 ## State Priority
@@ -245,6 +262,7 @@ state_priority(important_dof)
 
 ```@docs
 state_priority
+ModelingToolkit.VariableStatePriority
 ```
 
 ## Units
@@ -261,6 +279,17 @@ getunit(speed)
 ```@docs
 hasunit
 getunit
+ModelingToolkit.VariableUnit
+```
+
+## Variable type
+
+This metadata is used by the [`System`](@ref) constructor for automatically identifying the different types of variables in a system.
+
+```@docs
+ModelingToolkit.VariableType
+ModelingToolkit.MTKVariableTypeCtx
+ModelingToolkit.isparameter
 ```
 
 ## Miscellaneous metadata
@@ -277,6 +306,7 @@ getmisc(y)
 ```@docs
 hasmisc
 getmisc
+ModelingToolkit.VariableMisc
 ```
 
 ## Dumping metadata
diff --git a/src/ModelingToolkit.jl b/src/ModelingToolkit.jl
index 1dbf9d37dd..26efc0c7a9 100644
--- a/src/ModelingToolkit.jl
+++ b/src/ModelingToolkit.jl
@@ -134,7 +134,7 @@ in a non-breaking release. Usage of these arguments is not advised.
 """
 $(TYPEDEF)
 
-TODO
+Abstract supertype of all system types. Any custom system types must subtype this.
 """
 abstract type AbstractSystem end
 # Solely so that `ODESystem` can be deprecated and still act as a valid type.
@@ -336,6 +336,15 @@ export DynamicOptSolution
 @public is_diff_equation, Equality, linearize_symbolic, reorder_unknowns
 @public similarity_transform, inputs, outputs, bound_inputs, unbound_inputs, bound_outputs
 @public unbound_outputs, is_bound
+@public AbstractSystem, CheckAll, CheckNone, CheckComponents, CheckUnits
+@public t, D, t_nounits, D_nounits, t_unitful, D_unitful
+@public SymbolicContinuousCallback, SymbolicDiscreteCallback
+@public VariableType, MTKVariableTypeCtx, VariableBounds, VariableConnectType
+@public VariableDescription, VariableInput, VariableIrreducible, VariableMisc
+@public VariableOutput, VariableStatePriority, VariableUnit, collect_scoped_vars!
+@public collect_var_to_name!, collect_vars!, eqtype_supports_collect_vars, hasdefault
+@public getdefault, setdefault, iscomplete, isparameter, modified_unknowns!
+@public renamespace, namespace_equations
 
 for prop in [SYS_PROPS; [:continuous_events, :discrete_events]]
     getter = Symbol(:get_, prop)
diff --git a/src/parameters.jl b/src/parameters.jl
index de7a722af3..7633d8c1b7 100644
--- a/src/parameters.jl
+++ b/src/parameters.jl
@@ -1,9 +1,27 @@
 import SymbolicUtils: symtype, term, hasmetadata, issym
+
+"""
+    @enum VariableType
+
+The type of the declared variable, used for automatic identification of
+variables/parameters/brownians/etc. by the `System` constructor.
+"""
 @enum VariableType VARIABLE PARAMETER BROWNIAN
+
+"""
+    $TYPEDEF
+
+The symbolic metadata key for storing the `VariableType`.
+"""
 struct MTKVariableTypeCtx end
 
 getvariabletype(x, def = VARIABLE) = getmetadata(unwrap(x), MTKVariableTypeCtx, def)
 
+"""
+    $TYPEDEF
+
+Check if the variable contains the metadata identifying it as a parameter.
+"""
 function isparameter(x)
     x = unwrap(x)
 
diff --git a/src/problems/jumpproblem.jl b/src/problems/jumpproblem.jl
index 3bc0e11f61..32aa25182f 100644
--- a/src/problems/jumpproblem.jl
+++ b/src/problems/jumpproblem.jl
@@ -210,6 +210,13 @@ function get_variables!(dep, jump::MassActionJump, variables)
 end
 
 ### Functions to determine which unknowns are modified by a given jump
+
+"""
+    $(TYPEDSIGNATURES)
+
+Push to `munknowns` the variables modified by jump `jump`. `sts` is the list of unknowns of
+the system. Return the modified `munknowns`.
+"""
 function modified_unknowns!(munknowns, jump::Union{ConstantRateJump, VariableRateJump}, sts)
     for eq in jump.affect!
         st = eq.lhs
diff --git a/src/systems/abstractsystem.jl b/src/systems/abstractsystem.jl
index 3fe04936d3..f9d9a196b4 100644
--- a/src/systems/abstractsystem.jl
+++ b/src/systems/abstractsystem.jl
@@ -1095,6 +1095,12 @@ end
 renamespace(sys, eq::Equation) = namespace_equation(eq, sys)
 
 renamespace(names::AbstractVector, x) = foldr(renamespace, names, init = x)
+
+"""
+    $(TYPEDSIGNATURES)
+
+Namespace `x` with the name of `sys`.
+"""
 function renamespace(sys, x)
     sys === nothing && return x
     x = unwrap(x)
@@ -1141,6 +1147,11 @@ function namespace_guesses(sys)
     Dict(unknowns(sys, k) => namespace_expr(v, sys) for (k, v) in guess)
 end
 
+"""
+    $(TYPEDSIGNATURES)
+
+Return `equations(sys)`, namespaced by the name of `sys`.
+"""
 function namespace_equations(sys::AbstractSystem, ivs = independent_variables(sys))
     eqs = equations(sys)
     isempty(eqs) && return Equation[]
@@ -1162,6 +1173,11 @@ function namespace_tstops(sys::AbstractSystem)
     end
 end
 
+"""
+    $(TYPEDSIGNATURES)
+
+Namespace the given equation with the name of the given system `sys`.
+"""
 function namespace_equation(eq::Equation,
         sys,
         n = nameof(sys);
diff --git a/src/systems/model_parsing.jl b/src/systems/model_parsing.jl
index c24c063ee0..61edc394b2 100644
--- a/src/systems/model_parsing.jl
+++ b/src/systems/model_parsing.jl
@@ -186,7 +186,6 @@ function update_readable_metadata!(varclass_dict, meta::Dict, varname)
         (:description, VariableDescription),
         (:unit, VariableUnit),
         (:bounds, VariableBounds),
-        (:noise, VariableNoiseType),
         (:input, VariableInput),
         (:output, VariableOutput),
         (:irreducible, VariableIrreducible),
diff --git a/src/utils.jl b/src/utils.jl
index d028d4ed18..c6f3777c2b 100644
--- a/src/utils.jl
+++ b/src/utils.jl
@@ -88,9 +88,31 @@ function readable_code(expr)
 end
 
 # System validation enums
+"""
+    const CheckNone
+
+Value that can be provided to the `check` keyword of `System` to disable checking of input.
+"""
 const CheckNone = 0
+"""
+    const CheckAll
+
+Value that can be provided to the `check` keyword of `System` to enable all input
+validation.
+"""
 const CheckAll = 1 << 0
+"""
+    const CheckComponents
+
+Value that can be provided to the `check` keyword of `System` to only enable checking of
+basic components of the system, such as equations, variables, etc.
+"""
 const CheckComponents = 1 << 1
+"""
+    const CheckUnits
+
+Value that can be provided to the `check` keyword of `System` to enable checking of units.
+"""
 const CheckUnits = 1 << 2
 
 function check_independent_variables(ivs)
@@ -228,8 +250,23 @@ function iv_from_nested_derivative(x, op = Differential)
     end
 end
 
+"""
+    $(TYPEDSIGNATURES)
+
+Check if the symbolic variable `v` has a default value.
+"""
 hasdefault(v) = hasmetadata(v, Symbolics.VariableDefaultValue)
+"""
+    $(TYPEDSIGNATURES)
+
+Return the default value of symbolic variable `v`.
+"""
 getdefault(v) = value(Symbolics.getdefaultval(v))
+"""
+    $(TYPEDSIGNATURES)
+
+Set the default value of symbolic variable `v` to `val`.
+"""
 function setdefault(v, val)
     val === nothing ? v : wrap(setdefaultval(unwrap(v), value(val)))
 end
diff --git a/src/variables.jl b/src/variables.jl
index b61298eca2..46c9c95bc6 100644
--- a/src/variables.jl
+++ b/src/variables.jl
@@ -1,10 +1,45 @@
+"""
+    $TYPEDEF
+
+Symbolic metadata key for storing the unit associated with a symbolic variable.
+"""
 struct VariableUnit end
+"""
+    $TYPEDEF
+
+Symbolic metadata key for storing the type of connector that a variable is.
+"""
 struct VariableConnectType end
-struct VariableNoiseType end
+"""
+    $TYPEDEF
+
+Symbolic metadata key for storing whether a symbolic variable is an input or not.
+"""
 struct VariableInput end
+"""
+    $TYPEDEF
+
+Symbolic metadata key for storing whether a symbolic variable is an output or not.
+"""
 struct VariableOutput end
+"""
+  $TYPEDEF
+
+Symbolic metadata key for storing whether a symbolic variable is irreducible or not.
+"""
 struct VariableIrreducible end
+"""
+    $TYPEDEF
+
+Symbolic metadata key for storing the priority a variable has for being selected during
+state selection.
+"""
 struct VariableStatePriority end
+"""
+    $TYPEDEF
+
+Symbolic metadata key for storing miscellaneous information about a symbolic variable.
+"""
 struct VariableMisc end
 # Metadata for renamed shift variables xₜ₋₁
 struct VariableUnshifted end
@@ -227,6 +262,11 @@ function default_toterm(x)
 end
 
 ## Bounds ======================================================================
+"""
+    $TYPEDEF
+
+Symbolic metadata key for specifying the bounds of a symbolic variable.
+"""
 struct VariableBounds end
 Symbolics.option_to_metadata_type(::Val{:bounds}) = VariableBounds
 
@@ -436,6 +476,11 @@ function getbounds(p::AbstractVector)
 end
 
 ## Description =================================================================
+"""
+    $TYPEDEF
+
+Symbolic metadata key for storing the description of a symbolic variable.
+"""
 struct VariableDescription end
 Symbolics.option_to_metadata_type(::Val{:description}) = VariableDescription