Just a quick caveat: I asked this on the Ars Magica mailing list, but it seems we have devolved into talking about blinged-up bishops there and I was wondering about this issue still!
This might be mostly directed at the designers, if they are around, but anyone who has an interesting view please chime in!
I was wondering why auras impact on ever other mystical power (the Arts, Methods and Powers, supernatural abilities) but, aside from reducing the penetration score, do not inhibit the function of enchanted items?
I am curious about whether this is an oversight, a deliberate design decision to reduce complexity, a deliberate design decision to privilege enchanted items, a deliberate design decision to incorporate a pesky 'legacy concept' from earlier editions, or something else I haven't thought of yet.
Note that Might-based powers and magic item powers are both affected by auras in the same way: penetration only.
This is because penetration is really the only variable involved; everything else is set in stone when the powers of the item or creature are defined. Other things are more affected by auras because there's a die roll involved.
Which also means that if a supernatural critter (e.g. demon, through possession) does get inside your Aegis, your covenfolk are pretty much screwed, since they don't have MR and the Aegis only functions as MR for effects cast from the outside.
Patently false. The very reason why the RAW stipulates that tokens must be issued in order for guests to freely practice their Arts within another covenant's aegis is that it DOES act as MR for the area it covers (See box in top right corner of pg 69 ARM5:CR).
No, it reduces casting total for foreign mages and penetration totals for foreign creatures of might.
It only acts as a "barrier" type of magical resistance for spells cast from the physical outside.
This is all spelled out in the first two paragraphs of the spell description on ArM5 page 161. Note the "(originating from outside it)". I agree with Fruny.
That's your interpretation. My interpretation is that the box is a simplification of the full description in the spells chapter, and I'm sticking to it. YMMV.