Under rules of previous editions of Ars Magica, it had always seemed to me to be implied that the Parma Magica was a very specialised application of Rego Vim magics. Which is to say that Bonisagus' achievement was one of reducing the learning required to defend oneself against magic from a mastery of those arts to a simple and specific ritual. Though only inferrence, this seemed to make some sense to me.
By the rules of the new edition, however, magic resistance is no longer a function of Rego Vim. Is the magical basis of the Parma Magica discussed any further in any of the newer splatbooks, and if so what is it?
As a secondary question; does anybody know what the editorial rationale was for removing magic resistance from the Rego Vim guidelines for the fifth edition? It's one of the more significant changes to the magical rules, and I'd be interested to know why.
There are a few words in True Lineages, but essentially, it's just one of Bonisagus's great Breakthrough. It is not directly related to the Arts themselves. It is a Supernatural Ability that gets taught as part of the Hermetic tradition.
It increases the status of Parma Magica. It, and not so much the Arts, is the great secret of the Order of Hermes. Check out the end of the skill description on page 66 of the core book. Having universal magical resistance is a big deal. Allowing invested devices that provide magic resistance diminishes that substancially (though True Lineages offers them as a potential Breakthrough).
It was asserted that each "tradition" of magic would be allowed to break one of the limits of magic: Hermetic Magic breaks the limit of Universal Magic Resistance, Holy Magic seems to breaks the limit of Creation.
"Allowing invested devices that provide magic resistance diminishes that substancially"
That's not obvious. Magic resistance would still be a secret of the Order -- they could give away devices, but other traditions couldn't replicate or understand them.
But special casing Parma allows it to avoid the new warping rules for continuing mystical effects, by fiat.