This article is from the RCA SelectaVision VideoDisc FAQ, by email@example.com (Tom Howe) with numerous contributions by others.
All CED's have a number printed on the raised center section of the disc
that corresponds to either the number below the UPC (bar code) located
on the rear of the caddy or to the stock number printed on the edge
label of the caddy. The two numbers may not be exactly the same, but at
least four consecutive digits will match up. Side 1 of all CED's also
has the UPC encoded as a series of concentric striations on the raised
center section of the disc. RCA had machinery in the manufacturing plant
that would simultaneously read this engraved UPC and the printed UPC on
the caddy at the time of disc insertion to verify the correct disc was
being placed in the caddy with the correct orientation.