|Major||(post-Battle of Yavin)|
In the Imperial service, a major ranked between a commander and a colonel. In the Imperial Army and Stormtrooper Corps, majors were typically battalion commanders, leading units of approximately 600 to 1000 soldiers, while in TIE Fighter formations, they were often the executive officers of entire wings of 72 fighters, like Major Dalgas. They also acted as senior advisors and aides to colonels, generals, and Moffs. One of the most prominent Imperial officers to hold the rank of major was a clone of Royal Guardsman Grodin Tierce, who was in virtual command of the Imperial Remnant's entire military while advising Moff Vilim Disra and Flim during the Caamas Document crisis.
In the New Republic, the absorption of personnel from many different militaries with their own traditions may have caused a great deal of confusion in rank structure; but by the time of the Galactic Alliance, the rank of major had been firmly shifted down a grade to sit between captain and Lieutenant Colonel. In contrast to the Empire, Majors here were often found commanding smaller formations, for instance flights of a few fighters within a squadron unit.
In the Galactic Alliance Guard, however, the rank of Major was still borne by battalion commanders like Salle Serpa. This may indicate a revival or continuance of the Imperial Army usage, but it could also have been a response to the anomalous situation of the entire GAG, including Star Destroyer and numerous ground units, being commanded by a colonel rather than a general or a flag officer.
Behind the scenes
In real-life Anglo-American systems, a major ranks between a captain and a Lieutenant-Colonel, and this system is used in the later books of the New Jedi Order storyline. But the Imperial rank seems to be slightly higher, apparently equivalent to a Lieutenant-Colonel in US or UK forces. For instance, Turr Phennir was promoted from captain to commander in 3 ABY after the Battle of Derra IV, but was a major at the Battle of Brentaal in 5 ABY. This rank hierarchy corresponds to the historical French usage of the Napoleonic Empire.
The Rogue Squadron novels and comics are ambiguous, perhaps deliberately so, about the exact position of a major in the New Republic hierarchy—the positions of all the ranks between colonel and captain are left somewhat vague, but a passage in one of the novels shows that at least some majors outrank some commanders during 6 ABY. Several interpretations are possible, including that the normative rank structure of New Republic Starfighter Command up to the formation of the Galactic Alliance continued to conform to the Imperial system, with "major" equivalent to "Lieutenant-Colonel" and "(Flight) Commander" equivalent to an Anglo-American Major, albeit perhaps with an additional "Wing Commander" rank as an alternative to major.