A definition of stamps on stamps
There are three basic types of stamps on stamps: A, B, and U:
Type A – Stamp that contain (fully or partly) the original stamp (some times with modification).
Germany Scott: #1473
Type SY / U - Stamp that contain an unidentifiable form of stamp: stamp on an envelope, symbol of stamp, etc.
France Scott: #B400
Type B – Stamp that contain something of the original design or that use the same design with modifications.
Australia Scott: #229a
As said, this is the basic definition, and one can find other types of stamps which can be part of the stamps on stamps collection.
You can read more in an artical by Bunny Kaplan: STAMPS ON STAMPS: THE LOGICAL TOPICAL