Barcodes are sometimes found in the "matrix area" of CD and DVD discs.
SRC began using a Code 39 barcode sometime in 1990 and continued until the Cinram merger in 2003. However, many discs pressed from mid-1992 to mid-1993 do not have a barcode, and after that, the blocks in the barcode were narrower/thinner than before. The barcode apparently represents a sequential project code assigned at the time of mastering—perhaps a premaster ID. It can sometimes begin or end with a letter. The barcode on represses retains the same project number as previous pressings, even if they didn't have a barcode, so it is possible to use a repress to infer an approximate release date for an original pressing. On rare occasion, the barcode is not scannable because it is missing the stop bars at the end.
WME apparently began using Code 39 barcodes around 1988. At some point they switched to Code 128, which continued through the late 2003 merger with Cinram and into the present day. The Code 128 value is often the same as the matrix number in the human-readable part. The Code 39 value is sometimes a more cryptic set of 8 alphanumeric characters like 045D8SHK or 03H5RPK8, and is sometimes (often?) not scannable because it is missing the stop bars at the end.
Many DADC discs have a narrow barcode. The format has yet to be identified and decoded.
Some JVC discs use a block code which has yet to be identified and decoded. Each character in the code is a 3x3 pattern of blocks. The code is difficult to scan or photograph.
Some US releases on Epic look like they were made on the same kind of gear as the WEA Manufacturing / SRC discs of the early 1990s, but no manufacturer is credited. The barcode is thick-block Code 39 and (as seen so far) just contains the catalog number, without spaces.