In the BFME games, Acquiry is the process of targeting an enemy unit or structure and taking action. With the one exception of Orc Laborers, heroes, combat-units, and siege machines who are not in a Defensive Stance automatically acquire enemies that come within their vision range, and attack them.
Auto-acquiry for heroes and non-siege units will not occur when they are being fired upon by arrows, unless the enemies firing at them are within their sight.
Defense towers and siege-machine expansions, which both do not have Stances, also auto-acquire enemy units and structures. However, the auto-acquiry of siege machines (whether expansionary or mobile) is able be toggled by the player, because of the crush-damage their projectiles could inflict on friendly infantry as well as enemies, and also to control the hazard of resulting ground-flame from Siege machines that fire flaming rock. As an alternative to auto-acquire toggling, Ents, Mountain Giants, and rock-throwing Trolls, which are not siege machines, can be told to use melee attacks.
The only way to make heroes, infantry, cavalry, and non-siege beasts not automatically acquire enemies is to press 'G' with them selected, which puts them in their Defensive Stance.
Ranged units standing on rampart-walls who see an enemy they are unable to attack will move to where there is no obstruction, even if it involves descending from the wall. Ranged units other than siege machines who are fully enclosed within normal walls will not acquire enemies who come within their vision radius, unless those walls have a postern gate installed.