Infiltrators are skilled at sneaking forwards to scout out the enemy before the main force arrives. By making the best use of available cover, advancing at night or with other troops providing diversions, they are able to deploy ahead of the rest of the army.
The way that this is represented in the game varies from mission to mission, but will typically involve placing the Infiltrators last or allowing them an extra move in the turn they move onto the battlefield. Often only one side may use its infiltrating troops fully as their enemy is caught off guard; the missions will specify when this is the case. If Infiltrators are not specifically mentioned then they simply deploy with the rest of the troops, the circumstances of battle not allowing them time to use their special skills.