Codify the meaning of "undefined" and "implementation-defined" #3775
Description
We already use these terms a bit in the spec, but with PRs like these:
- Limit interoperable parsing expectations (avoid type conflicts) #3732
- Clarify interoperable parsing expectations (3.0.4 port of #3732) #3772
- Clarify how Schema Objects require full-document parsing (3.1.1) #3758
we are relying more on such descriptions to define areas of interoperability and expected compliance. A new subsection under the Definitions section seems like the best approach.