Once the defender has placed all his hidden set up markers the attacker then deploys his forces as described in the Scenario Set-up rules. Once the attacker's deployment is complete the defender reveals his hidden set up markers and places the appropriate models on the table. For a unit of troops or multiple vehicles one model is placed on the counter with the rest of the unit in 2" coherency as normal. A lone vehicle must be placed on the counter itself. Counters which are for units in reserve have a chance of being a minefield or a booby trap.
Revealing Hidden Troops
Source: Core Rulebook 3E, p. 134
Once the attacker finishes deploying, the defender reveals hidden set-up markers and places the real models in their place.