substitutions example

Author: Chris Cho

source/style-guide/markup/format/substitutions.txt

@@ -8,6 +8,12 @@ source constants. When used consistently, they help prevent inconsistencies in
 formatting the term and make it easy to change the value for all its
 occurrences.
 
+For example, you can use a substitution to represent the name of a MongoDB
+product, like Compass. with a variable, descriptively named ``gui-tool``.
+If the product name changes, you only need to update the value of the
+variable ``gui-tool`` to update all the instances of "Compass" in the
+documentation site.
+
 Source Constants
 ----------------