ISRC's explained

What are ISRC's?

ISRC's are an international standard used to uniquely identify sound recordings and music video recordings.

Important

Make sure you use the same ISRC code anytime you re-release a track that has already been previously assigned an ISRC code. The exception to this rule is if the re-released track has been edited, remixed, remastered, or manipulated in anyway. In the case for remixes, edits, remasters, etc, a new ISRC should be issued. Otherwise, always use the same ISRC if this particular version of the track has ever been released previously via any means, whether by a different distributor, or on another single, compilation, mix or anywhere. This is important for publishing and tracking purposes.

To be clear about the above rule, anytime a different master, edit, or version of a track is made, the old ISRC should not be used -- a new ISRC should be created to represent the the new version. You would only use the old version again if you were re-releasing the original unedited master again (for example in a compilation).

More info about ISRC:

The International Standard Recording Code (ISRC) is an international standard code for uniquely identifying sound recordings and music video recordings. —Wikipedia