Winscrn -
It used to be a convention that if you had either 5000 hours, an ATPL of some description or a Training (IRE) qualification, you were entitled to call yourself Captain. The trouble was that as smaller airlines became popular, they didn't have people so qualified and passengers were wary of flying with pilots who didn't have the shiny stuff on their sleeves. Thus, the various rank gradings have become blurred and you're a commander (small c) if you're in charge of any aircraft, in the same way that people in charge of smaller seagoing vessels are Ship's Masters, as opposed to Captains.
Phil