Good question, since neither KI nor HIT actually used those ranks
It's up to each general to say what these mean. Here's what they meant back in BF42, when we started using these ranks.
General -> the ultimate leader of an army, Shrapnel and Hitman last time
Colonel, LtCol, Major -> the high command. They do more field commanding, map strategy, campaign strategy, and organizing offline
Captain -> overall leader of a squad, responsible for organizing guys in that squad offline and leading it in battle. Often step up to field command.
Lieutenant -> backup leads of a squad, help run certain aspects of it, lead when the captain isn't there or doesn't want to. Often newer players in training
Sergeant -> not technically an officer, meaning no official responsibilities in the forums or outside battledays. Don't usually field command, but can squad lead when needed especially if splitting up into 4-man crews.
A "fully stocked" officer corps might have:
- 1 general
- 3 high command, w/ different responsibilities. Eg one risk guru, one guy who runs practices, one frequent FC, etc.
- 4 squads (for example, infantry infantry armor air) each with
- 1 captain
- 2 lieutenants
- 2 sergeants
But like I said, in practice generals can organize however they want. They might choose to leave some slots empty so they have more room to promote later, or change the number of squads they have, or not have a "high command" at all and instead put everyone in a squad.