 The ISSN applies to the entire publication, however, including every volume ever printed, so this more specific identifier was developed by the Serials Industry Systems Advisory Committee (SISAC) to allow references to specific parts of a journal.The variable-length, free of charge, code is compatible with other identifiers, such as DOI, PII and URN.Prior to January 2009, SICIs were valid DOI suffixes for registration at the CrossRef registration agency.