The root schema is the schema that comprises the entire JSON document in question.

This doesn't seem to make sense in the context of i.e. OpenAPI. The entire JSON document in question in that case would be the OpenAPI specification document. Maybe we should say something like "The root schema is the top-level schema resource for the schema in question"?

Note that documents that embed schemas in another format will not have a root schema resource in this sense.

Is this true though? If I have an OpenAPI specification document where on a request input I defined a schema with potentially a nested schema resource, wouldn't I call the top-level schema from that request input the root schema resource for that specific JSON Schema?

The root schema is the top-level schema resource for the schema in question

Then you have a problem with the next sentence that says that a root schema is always a resource. This is to allow for the case where a schema embedded in some other document (e.g. OpenAPI or the tests) wants to declare $schema but not $id.

You can't call it a resource yet because it hasn't been declared to be one yet.

It all does need to be massaged out.

Ah, good point. It does need clarification indeed.

I think this was likely cleared up by #1537.

@json-schema-org/spec-team can someone verify, please?

can someone verify, please?

That text hasn't changed. The issue still exists.