210 Subfield $2 - Source:

Subfield $2 contains a MARC code that identifies the source list for the abbreviated title, if the source is known. It is used primarily when the second indicator position contains value 0 . It may also be added when the second indicator contains value blank and another abbreviated title list uses the same abbreviation as the abbreviated key title. The source of the MARC code is the MARC Code List for Relators, Sources, Description Conventions that is maintained by the Library of Congress.

Examples:

210 00$aJAMA$2dnlm

210 0#$aJAMA j. Am. Med. Assoc.