Uh, sorry, playing too much World of tanks makes me talk by acronyms

HE = High Explosive, as AP stands for Armor Piercing, HEAT for High Explosive Anti Tank, etc.


IFA, as stated above, means Impenetrable Front Armour

Sometimes a HE shell from a howitzer or a similar large caliber would tear a turret apart, damage the turret rotation mechanism or other equipments, or kill, injure or incapacitate the crew inside an enmy tank without penetrating the armor.

Anyway, this is to justify 'historically' the choice to not allow a tank with turret to play with the shielded front side of the IR receiver.