[relequestual]

It is not =]

Additionally, the value is a "schema" as opposed to a boolean, meaning you can apply a subschema to any additional fields. Useful if you for instance want to make sure any additional fields start with a specific prefix.

Booleans are valid schemas. It's subschemas all the way down! I wrote an article around this topic: https://json-schema.org/blog/posts/applicability-json-schema...