The term served to distinguish those who were captains by rank from:
- Officers in command of a naval vessel, who were (and still are) addressed as captain regardless of rank;
- Commanders, who received the title of captain as a courtesy, whether they currently had a command or not (e.g. the fictional Captain Jack Aubrey in Master and Commander or the fictional Captain Horatio Hornblower in Hornblower and the Hotspur); this custom is now defunct.
Once an officer had been promoted to post-captain, his further promotion was strictly by seniority; if he could avoid death or disgrace, he would eventually become an admiral (even if only a yellow admiral).
In the Royal Navy of the time, an officer might have a rank, but not a command. Until the officer had a command, he was "on the beach" and on half-pay. An officer who was promoted from commander was a captain, but until he was given a command, he was on half-pay. Once the captain was given a command, his name was "posted" in the London Gazette.
An officer "took post" or was "made post" when he was first commissioned to command a rated vessel — that is, a ship too important to be commanded by a mere commander. Unrated vessels could additionally in a few cases be commanded by post-captains. Being "made post" is portrayed as the most crucial event in an officer's career in both Forester's Horatio Hornblower series and O'Brian's Aubrey-Maturin series.
A junior post-captain would usually command a frigate or a comparable ship, while more senior post-captains would command larger ships. An exception to this rule was that a quite junior post-captain can be posted to command an admiral's flagship, which was almost always a large ship of the line. The admiral would usually do this to keep his most junior captain under close observation, and subject to his direct supervision. Captains commanding an admiral's flagship were called "flag captains". One example of an admiral appointing a junior post-captain to command his flagship in this way is the appointment of Alexander Hood to the command of HMS Barfleur, flagship of his brother, Admiral Sir Samuel Hood.
Sometimes, a high-ranking admiral would have two post-captains on his flagship. The junior of the two would serve as the flag captain and retain responsibility for the day-to-day operation of the vessel. The senior of the two would be the fleet captain, or "captain of the fleet", and would serve as the admiral's chief-of-staff. These two captains would be listed in the ship's roll as the "second captain" and "first captain", respectively.
After 1795, when they were first introduced on Royal Navy uniforms, the number and position of epaulettes distinguished between commanders and post-captains of various seniorities. A commander wore a single epaulette on the left shoulder. A post-captain with less than three years seniority wore a single epaulette on the right shoulder, and a post-captain with three or more years seniority wore an epaulette on each shoulder. (National Maritime Museum Uniform Collection, see below.) In the O'Brian series, Aubrey "wets the swab" -- that is, he celebrates his promotion to commander and the acquisition of his "swab" or epaulette with the consumption of copious amounts of alcohol.
Note that the term was descriptive only: no-one was ever styled "Post-Captain John Smith".