During the Declare Battle Formations step, if every model in a unit has this ability, you can set it up in Reserves instead of setting it up on the battlefield. If you do, in the Reinforcements step of one of your Movement phases you can set up this unit anywhere on the battlefield that is more than 9" horizontally away from all enemy models.
At the end of your opponent’s turn, if this unit is not within Engagement Range of one or more enemy units, you can remove this unit from the battlefield and place it into Strategic Reserves.