Thanks forgot about that. I find it funny it mentions "cannot be modified by a weapon's Armour Piercing value", because I don't think they can be modified at all? From a streamlining point of view I wish the term "field armour" could have set of fundamental rules like you point out, but they are all over the place (as usual) and includes stuff like hexagrammmic fetish which isn't a "save" at all but instead gives negative modifier to enemy psyker when targeted.