An ISRC registrant should not assign ISRCs to recordings owned by another party.

ISRC should not be assigned if an ISRC has previously been assigned to the same recording. The existing ISRC should be used.

ISRC enables specific recordings to be unambiguously identified. When linked to reference metadata about rights ownership, ISRC can increase accuracy and efficiency in management of rights and other commercial aspects of recordings. The existing ISRC of the recording should be retained and used if the recording is licensed to another party or used across different countries.

ISRC identifies a recording regardless of format or bit-rate differences. The ISRC of a recording should remain the same across different releases or formats.

ISRC is not used to identify compositions or lyrics, nor is it used to identify the products or releases (such as a CD or an MP3) that may contain an embodiment of a sound recording.

ISRC is used to identify sound recordings and music videos only. ISRC should not be used to identify other items, such as merchandise, artwork etc.
MIDI files are not sound recordings, and should not be assigned ISRC

In the case of remastering, circumstances may determine whether a new ISRC is required or not. Details are available at Annex A of the ISRC Handbook.