ADVANCED POSITIONS
During deployment, when you set up this unit, if every model in this unit has this ability, it can be set up anywhere on the battlefield that is more than 9" away from the enemy deployment zone and any enemy models.
There is no stipulation allowing or requiring you use this deployment method after
normal deployment. You use it when you select to deploy that unit.
Most 9th Edition Mission Packs dictate when you place units into reserves, either Strategic Reserves or off the battlefield via a special rule. These generally have it done before unit deployments. Some special rules allow you to change a unit deployment during or after deployment, but generally you decide before any units are deployed.