Just talking with a member of my troupe, and the following came up:
If a spell with a (Form) requisite is cast on a Hermetic Magus, how does resistance work?
Eg1: Albertus casts Mind of the Beast (MuMe/An 30) on Beatrice - Penetration is 20 (AC at work here!)
Beatrice, being a jerbiton, has a mentem score of 15 as well as a Parma Speciality of Mentem, on top of her Parma score of 3.
However, her Animal score is 0. Her Resistance is 15 against animal, but 35 against mentem. What happens?
Eg2: Albertus appearantly like that spell, since he now casts it at Cerberus, penetration 12 (no AC here).
Cerberus has a Parma score of 2 (spec: Ignem), but an Animal score of 12 - thus Resistance 10 against Mentem, 22 against Animal.
Eg1: I'd say the same requisites count for resistance as for casting. If Animal resistance is lower than Mentem, use that
But allow me to make a further question: What about resistance for spells with a requisite, if the target has an Immunity to one of the two? Supposing a Magus with Greater Fire Immunity gets hit by a volcano spell CrIg(Te)?
If the requisite Art would affect the target, then all Arts must penetrate - so, in effect, the better/best of the defender's parma options is the one that defends. The spell effect relies on more than one Tech or Form to work as designed, so they all have to get through.
So, the MuMe(An) spell would have to penetrate either Parma+Mentem or Parma+Animal, whichever is higher.
If, however, the requisite is not involved in the effect - for instance, a ReMe(An) Sleep effect that can effect both Mentem and/or Animal targets, but in this case is being cast at a mage (Mentem only) - then only the Arts involved (here, Mentem, not Animal) need be considered. If on that same Mage's riding horse, to whom they've extended their Parma, then Animal and not Mentem. (And if the Target is Group, then the two, mage and Parma'd horse, might well have diff MR totals to penetrate.)
Lastly, I'd rule that Casting requisites don't matter - they are not a direct part of the spell effect, and they're already factored into a penalty if they are lower as a lower casting total (which is why they are "casting" requisites, and not spell req's.)
A perfectly valid interpretation as well We take that the requisite ios alread factored into the penetration total, so we only take the basic Mentem premise here, but I see CH's interpetation as valid as well.