[1] Reserves may be held back to defend against attack from other enemy forces, to be committed to the existing battle if the enemy exposes a vulnerability, or to serve as relief for troops already fighting.
[3] Reserves are kept and employed at all levels, from a platoon held back from a company level engagement, to whole army corps consisting of armoured and mechanised divisions which are held in reserve with the purpose of exploiting a breakthrough or containing an enemy advance.
Deciding where, how and especially when to employ reserves is a key command decision.
[2] In the event of reserves being sent forward to exploit a breakthrough, some are typically held back to deal with a potential counterattack.
[2] Reserves can also be employed to relieve troops in action, allowing those units to rest and regroup away from the front line.