Unstable units are Unbreakable, and suffer one extra Wound for every point they have lost of the round of combat by, with no saves of any kind permitted against these wounds. If an Unstable unit would normally be eligible to be Steadfast, they instead suffer 1 less Wound than they would normally do.
Unlike Unbreakable units, Unstable characters may join Unstable units. If an Unstable unit also contains Unstable characters, the controlling player first allocates wounds to the unit, then divides any remaining wounds (if any) as equally as possible amongst the characters.