In Twilight Imperium 4, when Space Cannon Defense fires during an orbital drop, which side has the right to allocate hit points?

Twilight Imperium Fourth Edition (TI4) ground invasion phase "Space Cannon Defense": The PDS on the defender's planet fires at the attacker's ground forces currently making an orbital drop and rolls 2 hits. Question: Are these 2 points of damage allocated by the attacker choosing two infantry units to die, or does the defender name specific mechs or infantry to destroy?