Does the ObjectID field have to be a Primary Key?

Does the ObjectID field have to be a Primary Key in an RDBMS (SQL for example)? I noticed in MS Access it isn't a PK.

I DO NOT use the ObjectID field for anything within our schema. It is strictly reserved for ArcGIS use. So, is there any reason it needs to be a primary key?

Any documentation that discusses the use of ObjectID as a PK would be appreciated.

G