If either side has any Deep Strike troops in reserve, then when they become available they may deploy anywhere on the table. This can be by teleporting, flying troops swooping down from the skies, creatures being summoned from the warp, tunnellers emerging from underground, or any other means of extraordinary movement.
To deploy Deep Strike units, choose a point anywhere on the board and place the centre of the Ordnance blast marker there. Roll a Scatter dice. On a 'hit' the marker is on target, on an arrow the marker is off target – move it 2D6" in the direction shown. Once the final position of the marker has been established, place all the Deep Striking models on top of it, facing in any direction. Deep Strike troops may not move or assault on the turn that they arrive, but they may fire as normal (though they count as having moved and so may not fire heavy weapons, etc).
If you roll a double 1 the squad has been lost – victims of a terrible accident, lost in the warp, shot down by enemy fire or crushed underground.
If the marker misses the table then the Deep Strike troops are not available for the rest of the battle.
Errata — Chapter Approved: Main Rules Clarifications (v2.0): Models deploying from Deep Strike cannot assault that turn, so must be placed at least 1" away from the enemy (you're not allowed to move to within 1" of the enemy except in the Assault phase). If any models are unable to do this, those models are destroyed.
Errata — Chapter Approved: Main Rules Clarifications (v2.0): When placing Deep Striking or Summoned models on the large ordnance template it is permissible to overlap bases by the minimum needed to fit the unit onto the template. Players must make every effort to fit models onto the template however congested they become. Sometimes however it may even be necessary (as a last resort) to place models beyond the edge of the template if the unit is particularly large. Under no circumstances should a player use this facility to get within shooting or charge range of a unit that would not otherwise be in reach and so they may only shoot or assault (if summoned, deep striking troops cannot assault) enemy units that can be reached by measuring from the edge of the template.