I think this is assuming an order of operations where you're determining the target's values first and then applying the attacker's effects. It seems from Helllois' answer (which was on Facebook unless I'm mistaken considering it's impossible to search as it'd have been posted as an image capture then) states that the order of operations is; determine the complete effect of the attack, then apply defensive skills. Both makes sense, but one has been convey by the English rulebook's editor that one of the game's community managers and rules editors have said is the correct way, while the other has not. You could certainly make a house rule or roll for it if your gaming group do not think the official answer is sufficiently searchable or if you don't agree with it, but from where I'm standing the official answer is known if not clear.
So, what about Shock or even DA/EXP, then? Is it also "tied to the ARM save" and can be avoided? (Rules > Rumours) (yes, I exaggerate a bit here)
Shock's effect is if you fail a "Saving Roll" you do not get to go into an Unconscious state. DA/EXP's effect is that you take more "Saving Rolls". Neither actually makes a difference if this is ARM or BTS or PH attribute used as AP does.
TL:DR we've been told that Bioimmunity has the following interactions: Against AP vs ARM attribute (i.e. Multi Weapons), it allows to use your full BTS value. Against AP vs BTS attribute (i.e. Breaker Rifle), it allows to use your full ARM value. Against Monofilament vs ARM, it allows to use your full BTS value. Against K1 vs ARM, it allows to use your full BTS value. Not an obvious interaction but that's how it's supposed to work. Example: Against a AP+EXP hit from a Missile Launcher a trooper with Bioimmunity with ARM4 BTS6 could use either ARM and have it halved by AP to ARM2 or his full unmodified BTS6.
These interactions definitely warrant a FAQ entry to clarify how they work; I anticipate many people stumbling over this.
The property of the ARM/BTS being tied to the weapon and special skills that allow you to choose bypassing effects like AP is absolutely not clear anywhere within the English rules. It would be better if this was a specific interaction under bioimmunity as it otherwise undermines the modularity of the ammo/trait system somewhat.
Oh, I'm not saying the rules are clear, I'm saying that the rules can be read in two ways and that the communications from CB have been made known which of the two is intended. We are absolutely waiting for official clarification through PDF edits or FAQ. Or even just a forum post in the solved rules questions subsubforum
