Is a subfield $b always required in a 670?
As noted in question no. 2 of this FAQ, the MARC 21 Authority Format requires a subfield $a; however, subfield $b is necessary only when information is being provided in support of the formulation and/or identification of the 1XX, 4XX and sometimes the 5XX fields. For example if the name as established in the 1XX is contained in the title of the item being cataloged (subfield $a), it is not necessary to repeat the name in a subfield $b. When creating or updating uniform title NARs it is seldom necessary to add a subfield $b to the NAR unless recording research. Nonetheless, in both these cases the inclusion/repetition of information in a subfield $b is not prohibited.
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