§ 807.2 Core Rules PDF (officieel)

If a Unit has Assault or has been granted Assault and is granted Assault by an additional source, the Assault Value of all granted Assault keywords is summed. Example: Petty Officer has Assault. It is chosen as the target of Cleave, which says "Give a unit [Assault 3] this turn." After Cleave resolves, Petty Officer has Assault 4 this turn.