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