The stone money of Yap : a numismatic survey / Cora Lee C. Gillilland. (OCLC #1255837)
The subject heading for Yap is:
Yap (Micronesia)
(direct order) so if that were the main subject of the title, that subject would be assigned in a 651. The authority record for that heading includes this field:
781 _0 ǂz Micronesia (Federated States) ǂz Yap
The 781 field is “Subdivision Linking Entry-Geographic Subdivision”, so indicates how that heading should be used as a geographic subdivision of a topical heading (indirect order). In this bib record, this appears as:
650 _0 ǂa Ethnology ǂz Micronesia (Federated States) ǂz Yap.