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.

If a unit with the Deep Strike ability arrives from Strategic Reserves, the controlling player can choose for that unit to be set up either using the rules for Strategic Reserves or using the Deep Strike ability.

Some units have ‘Scouts x"’ listed in their abilities. If every model in a unit has this ability, then at the start of the first battle round, before the first turn begins, it can make a Normal move of up to x" as if it were your Movement phase – as can any DEDICATED TRANSPORT model such a unit starts the battle embarked within (provided only models with this ability are embarked within that DEDICATED TRANSPORT model). A unit that moves using this ability must end that move more than 9" horizontally away from all enemy models. If both players have units that can do this, the player who is taking the first turn moves their units first.