Barcodes are sometimes found in the "matrix area" of CD and DVD discs.
The asterisks used as start and stop characters in some types of barcodes should not be entered; they are technically not representing asterisks, and scanners typically omit them when reporting the scanned value.
Scanners may not make it clear when there are leading or trailing space characters in the barcodes, and entering these may be difficult without resorting to using notes and substitutions.
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 tends to be exactly 8 characters (not counting the start/stop characters) and is either a variation of the human-readable part or a more cryptic set of 8 alphanumeric characters like 045D8SHK or 03H5RPK8. Sometimes the Code 39 value is 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.
Sonopress sometimes uses a Code 128 barcode with a variation of the human-readable matrix text.
Crest National used a Code 39 barcode with a 5-digit catalog/project number followed by a letter.
Disque Americ used a Code 39 barcode as early as 1988, with a 6-digit catalog/project number. Later, a 3-character code was used, matching the code before the catalog number in the human-readable text (55Y in the 2nd photo here).
Amtech used a Code 39 barcode with a variation of the catalog/project number.
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.