Promotion is career advancement.
Kombrig states for командир бригады (commander of brigade).
Traditional military ranks were abolished after the revolution. They were replaced by names of positions. Commander of platoon, brigade and so on. Despite initial intentions these new titles were often used as ranks loosely connected to the current position. Traditional military ranks were restored during WW2 or shortly before it.
I'm afraid, only real enthusiasts or historians can answer you questions.