Main Effort allows an enemy city to be targeted as an objective of the Armee and its Korps. Main effort chews up supply and administration. Normal is what it says. Normal is good for defending. Normal is also the only posture that allows the Armee HQ to be voluntarily moved to another city.
Rest is good for restoring supply and admin levels.
Korps in an Armee are reduced to skeleton levels - typically only two divisions. So, each Korps has either two or four options each turn. An Armee has three. There is no direct, player controlled movement of Korps. An experienced player in command of all three Army Groups or Theatres can complete a turn in two to five minutes.