While most of the Brotherhood's soldiers came with them from the Capital Wasteland, many inhabitants of the Commonwealth have also come under their wing; from former Gunners, to those who simply want revenge on the Institute and their synths, no matter who gets in the way.
Their ranks are (from lowest to highest): Initiate, Aspirant, Knight, Knight Sergeant, Knight Captain, Knight Commander, Paladin, Paladin Commander, Star Paladin, Sentinel, Sentinel Commander and Sentinel Prime.
Initiates are commonly found at low levels when traversing the Commonwealth, and when the player reaches higher levels, Aspirants begin to take their place as the "low level" Brotherhood ground soldier.
Knight Sergeants slowly start to replace Knights in Brotherhood of Steel areas, such as checkpoints, guarding the Prydwen, Waypoint Echo, etc. They do not have the appropriate paint for their T-60 power armor, instead having common Knight paint.
One of the strongest Brotherhood NPCs in the game, at early levels, they will often be found exclusively guarding the tarmac surrounding Liberty Prime in a full set of Sturdy Combat Armor of the Brotherhood variant.
A rare and extremely deadly Paladin breed, even without power armor they have deadly weaponry at their disposal, and may have several pulse or plasma grenades to disable power armor or quickly kill enemies.
Sentinels are mostly free to pursue their own gains for the most part, but are only called in to serve the Brotherhood on special assignment. They often possess heavily modified Laser Rifles or Gatling Lasers, that they use with deadly efficiency.
Sentinel Commanders are often a part of an extended personal project--- often leading other Brotherhood soldiers into new parts of post apocalyptic America. They are granted the rank of Sentinel Commander when they return with extremely successfull results. They have served the Brotherhood with distinction, valor, and honor for quite some time.
Most Brotherhood of Steel ground soldiers never make it to retirement. Which is why those who make it to Sentinel Prime are quite valuable to the Brotherhood for their combat experience, tactical knowledge, and ability to inspire other members of the Brotherhood in battle. Which is why the Brotherhood leadership prefers to keep them close at hand, though they are technically still ground troopers.