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KEEP: Creates a single index entry containing all of the MARC field's specified subfields, in the order in which they appear. Repeated subfields are kept in the same index entry as they appear in the field. Repeated MARC fields would get separate entries.
EACH: Creates a separate index entry for EACH iteration each copy of the subfield(s) within the single MARC field. Repeated subfields get separate entries.
REM: Creates a single index entry containing all of the MARC field's subfields, in the order in which they appear, EXCEPT for the specified subfields.
N/A: These fields have no subfields. Each complete field becomes a single index entry.
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