User blog comment:RemosPendragon/Questions to ask/@comment-33731050-20150226212728/@comment-24909831-20150227144641

The most accurate source for ranks that exists at the moment, in my oppinion, is the "Imperial Infantryman's Uplifting Primer". In it, there are couple ranks, let me quote you here:

^above list is direct quote from page 31, which clearly tells us that Lord Marshal, Captain General or Lord Castellan are not the same rank.
 * Command Structure of the Imperial Guard (Sector Level)
 * Lord General Militant (or equivalent)
 * General staff (Lord Marshal, Captain General, Lord Castellan and Staff Officers of Ranks from General to Lieutenant)
 * Regimental Commanders
 * Company Commanders

As said in the same book and page, here are the most prevalent ranks:
 * Warmaster
 * Lord General
 * General
 * Marshal
 * Lieutenant General
 * Major General
 * Colonel
 * Lieutenant Colonel
 * Major
 * Captain
 * Lieutenant
 * Sergeant
 * Corporal
 * Trooper

To me, honestly, it begins to seem that there is no official ranking system what so ever.