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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.
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).