In the topic on wards, Iudicium pointed out that the target of a PoF is the fire itsself, not the blasted individual. This came up in a discussion about penetration, but it also has other consequences. I don't want to ruin the very interesting other topic with it. So, here's a new topic:
The target of a PoF is the fire. That fire damages anyone it touches, let's say that this PoF does +20 damage.
A group target affects up to 10 individuals. It costs 2 magnitudes. Does that mean that a group PoF can do +20 damage, 10 times to the same person?
The target can of course soak this 10 times. Assuming a soak of 12, it still deals out 10 medium wounds, which is a -30 on rolls.
Adding 2 magnitudes to a PoF increases the damage by 10.
The target only soaks once, making it one incapacitating wound.
I don't know what's best, 10 medium wounds or 1 incapacitating wound. But the flexibility to choose how many people you want to blast and with how many PoF's makes the group PoF a must-have. (which in my book means that it disturbs game balance and should probably not work this well)
The difference here is that adding the size modifer does just that, it increases the size of the fire. What it doesn't do is increase the heat (and thus damage done) of the fire which requires a change in baseline. Thus increasing the target to Group does the same ammount of damage, but to everything in a bigger area. It changes it from a firebolt to a fireball. Increasing the baseline changes it, if you will, from the red-orange flame of a campfire to the cold, blue hiss of an oxygen torch. Thus for a level 25 spell, you can either do a fair bit of damage to a large area or a lot of damage to one thing.
They pretty much collapse to ash, yes. Multicasting can be phenomenally potent which, since to cast it five times you'd need a mastery score of 3 or 4 depending on speciality requiring many, many seasons of work, is only fair really.
A good rule of thumb is "One spell, one wound". (This has been discussed elsewhere recently - the alternatives spiral out of control quickly.)
But long before they have to bear that -30, they simply stop acting. So changing (as was my original example) Crystal Dart to a Level 20 spell "group", and expecting it to cast 10 darts (for 10 wounds, and the same crippling effect as you list), would imbalance many sagas.
But multicasting is just that - many spells.
As Fht says, expensive to get as high as you're talking about, but incredibly potent.
Ultimately, it's a "ymmv" sort of thing, but I find the logic persuasive.
The target "meaning thing created" (CrIg if i dont do mistakes for PoF): its the fire.
So the "target" is the fire. And you can do:
a fire = individual
a big fire that can basically hurt many people = group
a fire who takes all the place in a room = room
a very big fire = structure
a very great big fire = boundary
It's my opinion.
With all defaults.
Edit: after reflexions. I disagree with myself!
The target is the real target, but if you want to create a big fire, you can add magnitude for size (and, for a fire, that can also mean more damages).
Its more logically because for creo corpus, only individual and group are adequate target, like its said in the Core Book, somewhere
In my opinion, and ignorant of all the factors (caffeine, late nights, deadlines, lack of sleep, groupthink) that contribute to such, the choice of the word "Target" might be the worst decision since the use of the word "Levels" in a certain other game.
Casually speaking, when a mage chooses a victim (or more than one), they are (like it or not) the target (small "t") of his spell effect.
But with regard to that spell effect, in the spell design, when we're talking about Range/Duration/Target, the Target is always the Form used. The Spell has a Technique effect on the Target, the Form.
So, PoF has a Target of Ignem - even if the victim (the "target" with a small "t") is a bandit captain or pile of dry wood.
This can get confusing, especially with some forms (Ignem), direct damage spells, and some Intellego spells (where the Target is the Sense involved!), but if you follow those rules, and remember that Technique is applied to a Form, not the victim*, you should be fine.
(* With Forms like Corpus spells, these can be the same thing, by coincidence of the effect.)