From wiki:

Control Type: The strategy dictates which objectives the AI Commander prioritises first, the number of military groups used and their activity once they reach the objective.

Invasion: an invading commander will prioritise objectives closest to headquarters first (HQ is set at the location of the Military AI Commander module on the map). The commander will only use one or two groups to defend objectives and will focus on securing the next as quickly as possible until all objectives in its TAOR are secured or it runs out of groups to use. This is an aggressive and fast moving strategy but is vulnerable to attacks behind the front line.

Occupy: in this approach, the AI Commander takes into account a number of factors to determine the relative strategic importance of each objective including distance to HQ, the size and priority set by the Military Placement modules and the height above sea level. It will put a great deal of importance on holding objectives, sending up to 5 defensive groups if a location comes under attack. This is a cautious approach but uses a lot more troops to secure ground and so is vulnerable to attacks on the supply lines.

Asymmetric: insurgency commanders will attempt to cause maximum damage using as few military groups as possible. They will employ tactics such as sabotage, raids and ambushes whilst attempting to recruit fighters and coerce civilians into aiding their cause. Thorough patrolling and good intelligence is the only way to reduce the insurgency threat. More information can be found in the Asymmetric Warfare Briefing Note.