Universal Rule.
Rat Swarms cannot declare any Charges and always count as Fleeing when Charged (i.e. they must Flee as a Charge Reaction if able to, are destroyed if Pursued into, etc.). They cannot benefit from Commanding Presence.
A Rat Swarms unit can make a Sweeping Attack against a single unengaged enemy unit when passing within 1″ (it does not need to and cannot move through or over that unit). The enemy unit suffers 1 hit for each Health Point in the Rat Swarms unit with Strength 2 and Armour Penetration 1. If the enemy unit is Engaged in Combat, then randomise the hits as per the Callous rules.
If another unit (friend or foe) comes into base contact with a Rat Swarms unit, then that unit suffers hits as if. the Rat Swarms had performed a Sweeping Attack against it. The Rat Swarms unit is then removed as a casualty.