Barcodes are sometimes found in the "matrix area" of CD and DVD discs.
When decoded, the bars yield a string which is usually similar to some of the human-readable text found elsewhere in the matrix area. Sometimes the barcode instead stores an otherwise unmentioned serial number.
Scan/photograph the matrix area, warp the image to straighten the barcode, and then scan it from your screen.
Enter the barcode value in the Barcode and Other Identifiers (BAOI) section as a Barcode, with a description field saying where the barcode is, since the matrix area is an unusual location. The symbology can also be indicated, and a note if the release is known to also represent pressings without the barcode. Example descriptions:
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.
The ability to scan these codes raises questions of whether separate submissions for discs which vary only by the presence/absence or content of the matrix barcode. Discussion was attempted but reached no resolution. For now, they are not separate submissions, and it seems to working OK.
There are many more than this!
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, as they squeezed in another character in the same amount of space.
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.
Allied Record Company (ARC) used the same kinds of barcodes, but maintained their own sequence of project codes, and (based on what the sequence was at in 1995) only glass mastered about ¼ as many discs as SRC.
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 made in the USA or Austria since the mid-1990s have a narrow matrix barcode. Sometimes there is an additional short barcode on a separate layer of the molded plastic. The format has been identified and partially decoded by members of the Redump community, seen in this forum post.
From circa 1988 to 1996, discs from DADC's Pitman, New Jersey plant looked similar to the WEA Manufacturing / SRC discs of the early 1990s, including the use of a thick-block Code 39 matrix barcode. This barcode just contains the catalog number, without spaces. After 1996, the discs from this plant became almost, if not entirely, indistinguishable from those of the other DADC plants.
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.
Japan Victor Corporation (JVC) made CDs for the Victor and MCA labels, among others. Starting around 1987, JVC included in the matrix area a proprietary character set (not really a barcode, per se) which resembles 3×3 "Tetris blocks". The bottom row is always filled, and the top and middle rows represent the character's 6-bit ASCII code, with beginning and end markers using two or one fully filled grids. The images above show an example digitized barcode and a basic decoding guide. These discs are manufactured in a way that makes it very difficult to photograph or scan these codes.