There are a plethora of cases of regiments held in reserve during the battle or supposed to take part in an attack only to be recalled when things weren't looking promising.
As @Luke Freet mentioned, there were lot examples of regiment or brigades that didnt see action.
That is always a concern when Im trying to research where one of my ancestors fought. A soldier's service record never exactly says he was oresent in a battle even though his regiment was there. I always think maybe he was sent to the rear to get ammo. Maybe he was sick.
One turned out to be true. Despite a local marker that said my 2XGr-Grandfather was at a battle, his regiment was left 50 miles back to guard a bridge.
Keep that mind.