Provide the content’s standard identifier. Initially, this will likely be the ISSN
(presented with all 9 characters, including the hyphen and the check digit) or
ISBN (either ISBN-10 or ISBN-13, as available; link resolvers can convert as
necessary). In the future, this may include the ISMN, ISAN, and others.
In cases where multiple ISSN or ISBNs exist for the title, only the print-format
ISSN or ISBN should be used in this field.