It has been said that the hardest jump in the military is between colonel and general. That's because a colonel is a "specialist" who commands troops of only one type (infantry, cavalry or artillery), while a "general" (short for general officer) commands "combined" troops of different types. It was this fact, not the number of men, that made e.g. George Washington a four star "general." (At times, he commanded only 3,000-4,000 men, about the same as a colonel's modern regiment, and other "generals" of the time commanded around 1,000 men, about the same as a modern battalion commanded by a major or lieutenant colonel.
Likewise, a Navy "captain" (the equivalent of an army colonel) commands a single ship, by definition, of one "type."
The level between captain and admiral is called a commodore, who is the equivalent of a brigadier general. Does a commodore command multiple ships of the same type or different types? (A rear admiral commands the rear of a fleet, a vice admiral the front, and a full admiral a whole fleet, as a full general commands an army).
A"brigadier" general used to command a (combined arms) "brigade, when there were four regiments to a division, meaning two to a brigade, and two brigades to a division. Now that there are three regiments in a division, a "brigadier" has lost his original function.
Are brigadier generals and commodores TODAY "general" officers with independent commands as the term was originally understood? Or are they "training" positions for major generals and rear admirals to bridge the gaps between colonel and general, captain and admiral?