I recently started to play an Ex Misc learned magician as my magus in the Ad Fons saga, and as chartae seem to be this tradition's most impressive magic, he will probably use them for various situations. But, especially concerning chartae for himself and use within adventures, I started to realize, that I am not completely sure, how they function, and this makes a difference.
Possible concept 1: A charta just works basically like a charged hermetic item; the very moment it is finished the magical powers stored within are released, and from then on the piece of parchment doesn't matter any longer. This would mean:
a) The effect only has to penetrate the beneficient's Magic Resistance (in my case the Parma Magica) at the moment of the casting, so a magus can lower his Parma at the moment of the casting, accept the effect and then restor his PM.
b) Unless it is countered by some PeVi-like effects the enchanted effect will work throughout the duration fixed while producing the charta. This would mean it won't make a difference, whether I take the charta along, leave it or burn it.
c) If the recipient of the effect isn't touching the charta the very moment it is finished, all the work is wasted.
Possible concept 2: From the moment the charta is finished until the moment it expires it grants the effect it is made for to the person it was designed for, as long as this person is in contact with it. This would mean:
a) The target is only affected as long as the effect penetrates, so it is important to include enough penetration to counter stuff like the magus' own Parma or a negative dominion aura; otherwise the character might be forced to choose between his good luck charm and his Parma.
b) A charta's effect can be broken by removing the charta. It might even be possible to pick it up again, when needed, and restore the effect for the remainder of the duration. Especially, if the later is true, this would mean that they can be used without risking warping because of being constantly under a mystic effect, but this also makes it easy to break curses simply by a complete change of clothes.
c) I the recipient is not around when the charta is finished, this is no big deal. He will just be affected the very moment he finally touches the item. This might even be useful for the reason mentioned above.
Couldn't tell you what was intended, but parma does not dispel magic, so I think even if you're using model B there's an argument to be made that the recipient only needs to suppress for a moment (enriched things of virtue don't work this way so you may not win the argument, but you could make it).
This is the more appropriate option. The historical literature required that the chartae or amulet be in contact with the individual for it to have an effect. Maybe this should have been more explicit in the text, but it is described in some of the formulae. For example, the wound healing charta must remain in contact with the wound for the duration of the effect. The Blessing of the Crops charta must remain in the field for the duration of the growing season. I think there is a curse in there that states it must remain in contact with the target's dwelling for the curse to have effect, but this may have been cut in playtest, I don't remember.
As for your specific points:
a) A magus could lower his Parma Magica, don the charta, and raise the Parma while retaining the benefit of the device. But, Penetration still matters because a person could enter a hostile aura, which would temporarily supress the charta's effect.
b) The way this was handled historically was that chartae were not usually hidden on people, but used larger area of effects. For example, they might be buried on a person's doorstep or attached to the person's home or business.
c) A charta's clock starts ticking on the duration as soon as it is cast, the duration of the effect expires when it expires regardless of whether someone has picked it up or not. If the Target does pick it up, the charta affects him until the normal expiration of the charta. This is a meta-game mechanic. Historically, there was no problem with storing charta. The charta makers often kept many pre-made ones around to fill in the recipient's name as needed. The problem with this in Ars Magica is that it would encourage muchkinism, so we added the artificial expiration mechanic.
I am pretty sure that the above is what the author intended and may also even be supported by the rules as written.
Thank you for your fast replies, especially John's was naturally helpful. After the reading of the rules, version 2 surely had been the one which seemed to be implied. I was mainly unsure about the questions about belated activation, penetration and the possibility of turning the effect on and off by putting not permanently carrying the charm. Apart from the last aspect all seems clear now.
If I understood the rules correctly, the range is still just Touch (actually the only available range for chartae), meaning that the charta touches the house in question. This is a bit awkward, if you want to affect a specific person, as the target is Structure and everybody within the building is affected. I am a bit less sure, whether this means that troughout the duration everybody within the building is affected as long as they stay within the building, independent of the question whether they were there while the charta was put into its place, or whether those people who were in the buildung at that specific moment stay affected until the charta is removed, the effect disspelled or the duration is over, independant of the question, whether they stay in the building or not. I tend to the later interpretaion with the possible exception of wardings which should clearly affect the house and not the people within.