As written, D7.21 seems to be saying that one passed PAATC is sufficient to allow the infantry unit to attack the AFV as often as it is allowed during the rest of the MPh. It says nothing about anything changing if the AFV leaves and re-enters the hex. To me, this implies that one vehicle cannot force more than one PAATC by the same unit. If the unit doesn't need to pass a second PAATC to attack twice, it seems illogical that it might need to pass a second PAATC to keep its head down. A second vehicle would however force a second PAATC.
7.21 CC REACTION FIRE: Each Infantry/Cavalry DEFENDER unit that is unbroken, unpinned and neither Unarmed (A20.5) nor in Melee may attempt CC Reaction Fire, using the CC-vs-vehicle rules (A11.5, etc.; see also 7.211-213) [EXC: Ambush is NA unless using Street Fighting; 7.211]. After completing its attack, that DEFENDER and all of its possessed SW (including those Inherent) and Guns are marked with a CC counter, if the vehicle has survived, to prohibit non-CC Reaction Fire attacks, and also with a First or Final Fire counter as appropriate for that attack. To attempt CC Reaction Fire vs an AFV, the DEFENDER must first pass a PAATC unless it is exempt from PAATC (A11.6) or took one when the AFV entered its Location earlier in the MPh (A12.41). If it fails the Reaction Fire PAATC, it becomes pinned and can neither make that CC Reaction Fire attack (7.213) nor opt to make a Non-CC Reaction Fire attack (7.22) instead. A DEFENDER need not pass more than one PAATC to attack the same vehicle more than once during the same (A.15) phase.
