Library of Congress's ISSN page
It is an eight-digit number used as a unique identifier for serials or periodical publications, such as journals, magazines, newspapers
Many serials have identical or similar names, and the ISSN distinguishes one from the others.
For example, Rider has access to 6 different journals named Gazette. The ISSN helps us distinguish between two journals with the same name.
Each title of any published periodical has to have its own individual ISSN. Thus, If the title of the publication changes in any significant way, a new ISSN must be assigned.