From the API Version Testing design: "some of the commands that we'll convert to IDL have parameters that accept any BSON value; for example, createIndexes's "comment" parameter."
We therefore need an IDL type that can match + hold any BSON value. We originally planned to use the query system's Value type for this, but after discussing with them decided not to. See the discussion at SERVER-51875 for context.
This ticket tracks the addition of a new IDL type, or adoption of existing IDL type, to fit the need described. Will update description once approach is finalized.