I am guessing that this is the "unique record serial number" that allows easy merging of records that originated within the same PAF database. (This is useful when merging updates when several people are working on the same database, as in small extraction projects, and when several cousins update a single family database.) This represented a major advance in merging technology ... but I don't know how these numbers are generated.
Sorry! Maybe someone else will know. Gaylon Findlay sometimes monitors this forum, and as he did much of the programming of PAF, he would be the most likely person to have the answer.