A definition of stamps on stamps
Stamps on stamps refer to postage stamps that depict other stamps or philatelic themes. Some examples of stamps on stamps include stamps that feature images of famous stamps from around the world, such as the Inverted Jenny or the Penny Black, stamps that celebrate the history of stamp collecting, and stamps that pay tribute to notable philatelists or stamp designers. Stamps on stamps can be found in many countries and are often issued to commemorate special events or occasions.
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