This article is from the CD-Recordable FAQ, by Andy McFadden (email@example.com) with numerous contributions by others.
The term is used to describe a slight ridge near the hub of standard
CD-R media. This provides a small amount of separation between discs
stacked on a spindle. You can tell if your discs have stacking rings by
piling them up and then pressing down on the outside edge. If the stack
compresses slightly, they have the ring; if they're solid, they don't.
The ring is helpful when feeding discs into automated recorders because
it keeps the discs from sticking to each other. It can interfere with
hub labels or with printing near the disc hub, so you can often order
the same media with or without the ring.
There may be some benefit to using discs with the ring even if you're just
burning the occasional disc and using standard labels. The ridge is on
the bottom of the disc, which means if you put the disc down on a table,
most of the bottom surface won't be in direct contact. This could help