From the quite interesting article: > Captain Cheap, however, was promoted to post captain, […] If I understand the term post-captain correctly, this was not an actual rank to be promoted to. It just means that a captain's name was 'posted' on the Navy's seniority list as having been assigned command of a vessel, rather than holding the rank of captain without a command.
Post Captain was the permanent rank of captain, equivalent to the modern rank of Captain RN. Post Captains in the Royal Navy commanded ships of 20 guns and up, and the rank and title were retained when on half pay without a ship. Smaller sloops were generally commanded by a Commander (with the courtesy title of Captain when commanding; equivalent to Commander RN).
Critically, promotion to Post Captain was at the discretion of the admiralty, but onward to promotion to Rear Admiral and beyond was based strictly on seniority, so the Post Captain promotion was the critical one.