Follow-up ticket from
The RELS-EXT predicates isPageNumber and isSequenceNumber are used by Paged Content, Book, Book Batch, and Newspaper Batch.
isPageNumber should contain the identifier written on or contained in the page itself, for example, 7, iv, or C3.
isSequenceNumber is used to order items within the paged content model, must be a nonNegative integer (positive integer?).
I believe (though cannot prove) that isPageNumber was functional in Islandora 6, it is never used in any official Islandora 7.x code after being set. The fact that this predicate is non-functional ('deprecated'? 'vestigial'? 'unimplemented'?) should be communicated somehow, perhaps in the ontology.