Post captain as a substantive rank

How was a post captain different from an ordinary ship's captain?

A post captain held the substantive rank of captain, while an officer commanding a ship might formally be a commander or lieutenant commander.

A ship's commanding officer could be called its captain while holding the lower rank of commander or lieutenant commander. A post captain, by contrast, held the substantive rank of captain. The terminology could therefore describe either a command appointment or an officer's formal rank. This distinction helps decode rank and command language in Hornblower and other Age of Sail narratives.