The ISWC (International Standard Musical Work Code) is a standard promoted by CISAC, which identifies every musical creation as unique and intangible. Every time a musical piece is registered in a collective rights management company (Authors and Editors), the registry is associated with a unique ISWC code for each registered work.
The ISWC code identifies the piece and should NOT be mistaken for the ISRC, which corresponds to the recording (master). They are unique codes for each product.
The ISWC code is not mandatory. It is a registration code that can be added (or not) for informational purposes and does not create a conflict with digital distribution.