the SID term is unique or not? can two user sids on different machines in one domain system be equal? (if both of them domain users or if both of them locally users)
Two domain users can't have the same SID; its unicity is enforced by the domain (by the DC holding the RID Master FSMO role).
Two local user accounts on different computers shouldn't have the same SID, unless they're well-known accounts which always use the same SIDs (like Administrator); this would happen only if you created an account on a machine and then cloned it. It wouldn't matter much, anyway, as local SIDs are never used in network operations.