Cloudscape used to expose Virtual Table Interfaces, by which any class which implemented ResultSet could be included in a query's FROM list. Derby still exposes a number of these VTIs as diagnostic tools. However, Derby now prevents customers from declaring their own VTIs. The parser raises an error if a VTI's package isn't one of the Derby diagnostic packages.
This is a very powerful feature which customers can use to solve many problems. We should discuss the reasons that it was disabled and come up with a plan for putting this power back into our customers' hands.