When does Assault apply?

According to Rule 807.1, Assault is a passive ability that functions as: "While I am an attacker, I have +X [M]."

Key details regarding when it applies:

• Attacker Designation: Assault applies as soon as a unit gains the "Attacker" designation during combat.
• Passive Ability: Because it is a passive/continuous ability, it applies automatically and does not use the chain. You cannot respond to a unit gaining or losing its Assault bonus.
• Open Battlefields: Assault does not apply when moving to an open (unoccupied) battlefield, because a showdown on an open battlefield does not involve combat, and therefore the unit does not gain the "Attacker" designation.
• Combat Transitions: If you move a unit to an open battlefield (no combat) and the opponent later plays a unit there (e.g., via Ambush), the showdown transitions to combat. At that moment, your unit gains the "Attacker" designation, and the Assault bonus becomes active.
• Duration: Assault remains in effect as long as the unit maintains the Attacker designation for that combat.