By selecting a stance for a unit, you can specify how it should behave whenever it does not have specific orders.
Military Units[]
Military units are generally set to aggressive by default, but this can be changed for a unit production building or in game options.
Aggressive[]
Aggressive units will attack enemies on sight and will pursue them. While setting your units to be aggressive may not seem a bad idea, it can be annoying in some situations, especially when your army continuously becomes scattered and disorganized after just spotting one enemy unit.
Defensive[]
Units will pursue their targets, but only for a limited time before returning to their previous locations. This stance is good for patrolling units.
Stand Ground[]
Military units will still attack but they will not move to attack unless ordered to do so. This is excellent for your army if you prefer them to remain organized, although you may need to order them to move, especially if they are under attack and they cannot counter the enemy.
Raid[]
Units will attack (and chase) civilians as their primary targets. This is helpful to prevent enemy civilians from constructing buildings, such as a redoubt.
Raze[]
Military units will destroy enemy buildings as a primary goal. This is helpful in destroying military producing buildings (such as the barracks).
Hold Fire[]
Your units will never attack even if they are being assaulted by the enemy. This option is useful if your army is implementing an ambush behind enemy lines but this is not recommended to use if your units are being attacked.
Citizens[]
The stance for new citizens, along with the idle time delay, is controlled by a game option.
Idle[]
They will not move until they are ordered to do so.
Build[]
This will set workers to build as a top priority.
Gather[]
Workers will gather in empty farms, mines, woodcutter's camps, and oil wells.
Build and Gather[]
Civilians will build uncompleted buildings as a first task, then gather resources, then repair damaged buildings.