If your Army Faction is GREY KNIGHTS, at the end of your opponent’s Fight phase, you can select a number of units (see below) from your army that are on the battlefield (excluding units that are within Engagement Range of one or more enemy units), provided every model in those units has this ability. The maximum number of units you can select depends on the battle size, as follows:
"In an Incursion sized battle, you can select up to 2 units. In a Strike Force sized battle, you can select up to 3 units. In an Onslaught sized battle, you can select up to 4 units."
Once you have made your selections, remove those units from the battlefield and place them into Strategic Reserves.