These definitions from the Plate Number Coil Collectors Club glossary should help:
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Liner Paper
Almost all Self-Adhesive (SA) coil stamps have a backing paper or liner to which the stamps are affixed. There were a couple of experimental 'linerless coils' produced that did not employ backing paper. On coils of 100, the liner is the same size as the stamps. On larger rolls, the liner is often larger (taller) than the stamps. Some larger rolls also have space between the stamps on the liner.
Linerless Self-Adhesive Coils
USPS produced Linerless coils that use stamps as substitute liners. These experimental linerless coils were produced as a conservation measure to help preserve the environment. Examples are the 32c Flag and 25c Jukebox issued on March 14, 1997 and the 33c Berries issued in 2000.