Terminology is not 100% decided yet. See current forum threads at https://www.discogs.com/forum/thread/746274 , https://www.discogs.com/forum/thread/706164 and https://www.discogs.com/forum/thread/746182
What we are calling a distribution code in Discogs is a certain kind of code which appears on certain European releases in addition to one or more obvious catalog numbers.
We believe the codes are in some way tied to distribution and/or pricing, but we have no authoritative information about their actual origin and purpose. If you know anything about them, please add the info here.
We have observed that distribution codes do not always uniquely identify releases; the same codes are shared by multiple releases. Although there are counterexamples (some codes do seem to be unique), it seems best to rule that a distribution code is not considered a catalog number to identify the release itself, at least not for the way we use the catalog number field in Discogs. Therefore, the submission guidelines say distribution codes should only go in the Barcodes And Other Identifiers field.
In 2016, a Discogs user posted a guide to French codes which says they are price codes. See that guide for many details beyond what is presented here.
Here's a list of known distribution codes, their origins and the companies that they represent:
If a number printed on a record matches with one of these, it is usually a distribution code and should not be added as catalog#, but in the Barcodes And Other Identifiers field.
EMI enclosed the above type of distribution code within their 'international catalogue numbers' from 1969-1982*, and continued to display them on EU-pressed media until the mid-1990s prefixed with "F: PM..."† .
Rather than use a pair/triplet of letters to indicate the label (as with the above list), they used a pair/triplet of characters to indicate the country of distribution - one or two digits followed by one letter.
For "mid-price" re-issues the 'C' or 'E' was changed to 'M' (indicating either Mid-Price or being on the Music For Pleasure label, e.g.
This list is not exhaustive and codes were sometimes changed over time.
* Germany (also used by EMI in the Netherlands/Italy for continental Europe pressings)
Continued to use this system until 1991, displaying 1C ### before the rest of the catalogue number. In 1992 however EMI introduced a new 'catalogue number/bar code' box that tied all cataloguing information together (EAN, LC and distribution codes). Spaces were provided for French and German distribution codes (which could vary) prefixed F: and D: respectively (or on Italian releases, F: and I: - the latter number being the same as the D: one). At this point the 1C prefix (the last to remain in force) was dropped for the German distribution codes, and they fell out of use as the decade went on - first the German codes and later the French/Italian ones.