Please try reading the dev diary
"Now should you decide to assume manual control of a unit (say a Corps inside an Army Group) then this will deactivate the whole chain of command. Each existing unit will continue to follow its current order (e.g. move here or attack there) but no new ones will be issued. The AI cannot cope with human interference of its orders. "
Will theatres/army group areas of influence be user definable.
An example, Army group south (ai) is given France (Paris as a target and told to prepare) while I use army group east to attack Denmark and Norway as player controlled.