Disc Labels are the labels that are on AIO CD's.
Beginning with Album 16, albums have almost always been available on CD. Over the course of the series a few different designs have been used.
This is disc 4 from Album 42: No Way Out. This version has a picture and a color that varies per album.
This is disc 1 from Album 25: Darkness Before Dawn. This version is the current, with no picture (although there is a slight decoration).