When an allied Trooper unit with the or Rank is defending against a Ranged attack from an attacking unit whose unit leader is not within , the defending unit benefits from Backup if it is within and has LOS to the unit leader of an allied unit that provides Backup. A unit must meet the following requirements to provide Backup:

1. The unit has the Rank.
2. The unit is a Trooper unit.
3. The unit does not have a number of Suppression tokens equal to or greater than its .

A unit benefitting from Backup may cancel up to 2 results during the Modify Attack Dice step of the attack sequence.

Clarification: A unit with the Guardian X keyword cannot benefit from Backup and ignores the Rank requirement to provide Backup.[Source]

Abilities that ignore the corps Rank requirement: