$o

[pan-zhuo]

https://www.oclc.org/bibformats/en/7xx/76x-78x.html#subfieldo

"Numbers, codes, etc., used to identify an item when these identifiers are not appropriate for one of the more specific subfields (e.g., subfield ǂz). Item or accession numbers are examples."

Is $o only for item identifiers or both manifestation and item identifiers?

[CECSpecialistI]

Given the list of fields it can be used with, I think it could be an identifier for a work, expression, manifestation, or item. Whatever is being identified in the given field