There are a multitude of different systems for promoting Soldiers. Soldiers are promoted from E-1 through the rank of E-4 essentially at the whim of their commander with some minimum time in service requirements. Soldiers wishing to be promoted to Sergeant and Staff Sergeant attend a promotion board where they recieve points based on appearance, answers to technical and tactical questions, and bearing. These points are added together with points given by their Commander, points for demonstrated technical skills (Marksmanship and the like), points for awards/medals, and points for education. The Soldier then goes on the promotion standing list and waits until his points meet what we call the cutoff score. This score is determined by the Army based on the number of young SGT’s and SSG’s needed.
Soldiers achieving the rank of Sergeant begin getting evaluations, these are placed in their permanent file and, once a SSG (Staff Sergeant) reaches a certain time in service his packet goes before a central board in Washington. The Army then evaluates their performance record (evaluations), positions the Soldier has served in, and awards, civilian, and military education. The best are promoted from Staff Sergeant to Sergeant First Class, or E-7. The process is the same to get promoted to Master Sergeant (E-8) and Sergeant or Command Sergeant Major.
Officers are promoted in much the same way. A 2nd LT is promoted to 1st LT at about 18 months of service, this is more or less automatic. From the point a 1st LT stands a board for promotion to Captain all the way until he/she is boarded for promotion to General, the process is more or less the same as how SSG’s are selected to Sergeant First Class. Selection rates vary depending on the number of officers needed at the next rank.
All Service members at all ranks get a chance (or two) to be promoted early, on time, or late. Enlisted usually have more chances to be looked at for promotion, typically an officer is discharged or retired if he/she fails to be selected for promotion 3 times.
Battle field promotions for enlisted are authorized but aren’t really handed out by junior officers, typically it takes at least a Colonel to authorize the promotion. Battlefield commissions (promotion to LT) haven’t been authorized for years, but work in the same way with the exception that the commission isn’t permanent without approval from the Senate and President. Officers are appointed officials in much the same way a member of the Cabinet is. Battlefield promotions will sometimes happen for officers with the caveat that they are “brevet” rank, meaning a Captain can wear, at the convenience of the Army, the rank of say Lt. Colonel. He has all the rights and responsibilities (but usually not the pay) of said rank until such a time as he is no longer needed at that rank. He then reverts to his previous rank. Colonel Custer was a Brevet Major General during the civil war, he was reverted to Colonel when the war ended. Sometimes officers outright selected for promotion will be dialed back a rank or three at the conclusion of major conflicts when the Army draws down.
Hope this helps.