For a given entity, there often exist many ID fields within the record that could be used to construct a unique URL for that object.
What is the best ID to use when generating IDs for any entity?
Defining the appropriate URI generation scheme is up to the mapper and the institution. We recommendation to not use accession numbers for works of art, because they're not guaranteed unique, but there is no specific recommended best practice beyond choosing the most persistent, unique field.
(from Duane in email, Aug 16, 2016)
How object identifiers are defined (a separate topic from domains).
A separate issue has come up around specific IDs, particularly for objects. We know that some partners may use an Object ID (e.g. From TMS) while others may use an Accession Number. From the perspective of the Browse app (and other users of data), as long as it is unique, either one is OK. ISI may have specific URI or URL structures to recommend, but from our perspective they don’t all need to be similar.