Scouts are advance troops who sneak onto the battlefield in order to seize vital locations before the two armies clash.
Source: Warhammer Armies Project: Unofficial 9th Edition (v2.2)
ScoutsURL Copied!
Before starting deployment, a unit with the Scouts special rule can choose to not deploy at the start of the battle. Scouts are set up after all other non-Scout units from both armies have been deployed, and do not count toward determining who finished deploying their army first. They can be set up either in their controlling player's deployment zone as normal, or anywhere on the battlefield more than 12" away from the enemy. If deployed in this second way, Scouts cannot declare a charge in the first turn if their side goes first.
Note that a character may only join a unit deploying with the Scouts special rule if they also have the same rule.
If both players' armies contain Scouts, players should roll off. The players then alternate deploying their scouting units one at a time, starting with the player who won the roll-off.