Details on a Transformers toy that are not molded, printed, or painted on may be provided by the application of one or more stickers. Generally called labels, peel-and-stick labels, pressure-sensitive decals, or simply decals in polite conversation, none of these terms have achieved the popularity of the simpler vernacular.
These devices are simply a flexible substrate with colors and designs printed on one side, and some sort of gummy glue on the other. Generally they are stuck to a wax-coated protective sheet before use, and often the individual stickers are die-cut from a single sheet of backing while attached to this waxy sheet.
Backings used on Transformers include paper, plastics, and foil. Paper is inexpensive, but can not be transparent as plastics can. Foil is shiny shiny shiny fun, but can be stiff and reluctant to stay in contact with the decorated surface, in the long run.
Stickers were very common in Generation One, in which many toys came with a sheet of stickers and directions for their application. In more recent years they have largely given way to molded detail, paint applications, and tampography.
Most Generation One reissues of the original toys had their stickers removed and replaced by printed designs or paint application.
A toy's stickers are a common area for early wear, as the designs fade and are abraded away. Because of this, aftermarket manufacturers have sprung up that supply reproduction stickers. Some are available that were designed and never used on the original toy, and still others are original designs, supplied as improvements, to provide detail not previously found.
Stickers are fun and cool. It's totally worth studying for that test if you get a fuzzy doggie sticker by the "B+".