The ISWC (International Standard Musical Work Code) is a standard promoted by CISAC, which identifies a musical work as a unique intangible creation. Every time a work 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 identifies the work, and should NOT be confused with the ISRC that corresponds to the recording (Master). They are unique codes for each product.
ISWC is not mandatory. It is a registration code that can be added (or not) for informational purposes, and does not affect the results of the digital distribution.