What an ISRC is, exactly
An ISRC is twelve characters that permanently identify one recording: two letters for the country of the issuing agency, three characters for the registrant who was allocated the prefix, two digits for the year the code was assigned, and five digits chosen by the registrant. It is often written with hyphens for readability, but the hyphens are not part of the code. There is no check digit, so a mistyped ISRC is structurally valid and silently wrong — which is precisely why it is worth checking the structure and then checking your own records.
One recording, one code, forever
The ISRC belongs to the recording, not to the release, the territory or the owner. If the recording is licensed, sold or re-released, the ISRC travels with it unchanged. What does need a new ISRC is a genuinely different recording: a remix, a radio edit, a remaster, a live take, an instrumental, a sped-up version. Each of those is a separate recording with its own performance and its own royalty stream. Reusing one ISRC across several versions pools their streams together and makes the reporting impossible to unpick later.
What the barcode is for
The barcode — a UPC-A of twelve digits or an EAN-13 of thirteen — identifies the release as a product. A single, an EP and an album each need their own, and a track that appears on both a single and an album is one recording with one ISRC sitting inside two products with two barcodes. Unlike the ISRC, a barcode has a check digit, so a typo can actually be detected. That last digit is computed from all the others.
How the check digit works
Take every digit except the last. Read them from right to left and multiply alternately by three and one, starting with three on the rightmost. Add the results. The check digit is whatever you need to add to reach the next multiple of ten — or zero if the sum is already a multiple of ten. A UPC-A becomes the equivalent EAN-13 by adding a leading zero, and the check digit does not change. This tool runs that calculation, and if your check digit is wrong it tells you what it should have been.
Where to get your own codes
Most distributors will assign both for free, which is convenient and fine for most artists. The alternative is registering directly with your national ISRC agency for your own registrant prefix, which costs something and means the codes are yours regardless of who distributes you. If you are building a catalogue you intend to keep and possibly move, your own prefix is worth the paperwork. Either way, keep your own list of which code went to which recording. That list is the only thing that makes a catalogue auditable ten years later.