Hm, that's a really good point.
Tellus' point seems valid to me. And we interpreted the rules in the same way from the beginning. But each his own way.
Our group has gone farther: we banned all "sustaining" ReVi spells. We felt "sustaining" is merely another word for extending the duration of a spell, and this should be MuVi. In fact, when we began playing 10 years ago, most mage had his version of sustaining the demanding spell (often with duration: sun), using it on our spontaneous spells with great effect...too great effect, in fact.
The initial spell of this thread, Circle of the Demanding Spell Sustained, seems perfectly fine to me btr. I like the idea of targeting many spells with a Target:circle, haven't tought of that.
Hmmm... that last is an assumption, but not an unreasonable one. The RAW don't specify such a distinction, but they could be read as inviting it. Hrmmmm... indeed... 
I'm not sure I accept it, but I can't say that I dislike it.
I'm not sure that both would not work - the only (apparent?) diff is in magnitude, but nothing btr (that I can find) clearly states that one or the other is more/less in/appropriate.
While this sounds reasonable and almost convinced me, it does run contrary to the way the term "sustain" is (repeatedly) used in the Spell Binding mystery (TMRE) as a synonym of "extend" [a spell beyond its normal Duration].
Different author.
(Yeah, a word as used in a later work does not define a word as used in a previous work - unless that word is specifically being (more clearly) defined as a Game Term. This ain't that.) 
It does not really matter. It's still Ars Magica canon, and it ruins the argument about sustaining being different from extending.
No, ez - it's not. There is nothing in canon that states "Every use of 'sustain' is synonymous with 'extend' ", nor anything remotely close. Much less a statement referring to [i]previous[/u] uses in other books. An interpretation of that meaning within that one book is just that - an interpretation. Sorry, but - no.
Let me make myself clear.
There is canonical evidence that "sustain" is used as a synonym of "extend".
There is no canonical evidence that the two are ever used with a different meaning, since the only point where they may be is indeed the controversial one we are discussing.
By Occam's razor, all else being equal, it's just simpler to assume that the same word means the same thing throughout the game line.
Unfortunatly, they are being used synonimously by different authors, who may very well not have been aware/noticed the slight difference smply becaue it is only relevant in this specific context. At most, this proves nothing and leaves us back at square one, that we know nothing.
Unfortunatly Occam's razor is rather invalid, being an argument rutinely disproven in high schools.
Points for being in the medieval paradigm though.
It's not clear what you mean with "at most".
It certainly proves that we cannot assume that "extend" is different from "sustain".
It suggests that we can assume the opposite, since it's more reasonable to assume that (even different) authors are consistent than inconsistent, all else being equal.
Actually, Occam's razor is one of the foundations of modern scientific thought, and is constantly used throughout science. Of two theories that make the same verifiable predictions, the simpler theory is to be preferred. You will not find a single scientist that will disagree with that. Occam's razor cannot be proved or disproved, since it's not really within a formal system; but there are strong arguments "in favour" of it in information theory.
See the above statements by Cuch, please.
I rather believe I can find quite a lot of scientists who disagree with this.
The ones who trained me.
The ones that eg. understand that quantum mechanics is a nice piece of math that we're building on, but the accepted understanding of said math is little more than a game of charades because all we have is the math - if multiple theories explain the math and we have no way of checking them against eachother, then these theories are ultmately equally valid.
The ones that advice against using Occam's razor, because the probability that you have not fully analysed the situation and will thus mis-apply said razor is greater than the opposite.
Even if we choose to accept the applicability of the razor, we are not ready to wield it, because we do not know yet whch solution is the simplest.
Furthermore, we have no real data to compare to - the whole situation if fictional and the only spell we have which uses the guideline has been mentioned above. We cannot test the guideline, we can simply hope for a declaration of 'fiat' by the Line Editor.
Even if a supplement turns up stating one thing or the other, this will be a decission that cannot be assumed untanted by this discussion.
Tellus you were doing so well too...
They still use Occam's razor, neh?
They just take care about when to apply it.
Though I agree that the scientific method (build theory, test it, re-jig´theory, test, repeat ad nausam) is in applicable.
Ezz: they do have a point - odds that the author of the spirit binding text considered this specific context are slim to none.
And for every situation I can think of besides this, the difference is moot.
Personally I like Tellus' version. Gives more purpose to MuVi and you can't just re-jig every spell to be D: Ring.
But that's because I apply Occam's Razor to the outcome I guess 
And finally, a question:
To use D: Ring, everything targetted has to be inside the ring, neh?
Where is the spell?
Magus A casts a spell on grog B.
Which one of them have to be inside the Ring? Both of them?
Can you give canon spells that extend Duration using a "sustain" Guideline? Any of those for MuVi/ReVi?
Or is this "sustain" usage unrelated?
The Spell Binding msytery allows one to "bind" a spirit to a spell so that the spirit will allow the spell to "outlive" its normal duration. Throughout the description of the mystery the term used is "sustain the spell" instead of "extend the spell's duration".
I would point out that this is as far as it can get to the corresponding ReVi guideline. In both the guideline and in the mystery, one is not "extending the spell" (which would be unclear), nor even "extending the duration of the spell" (as one would do with a MuVi effect, that re-shapes the spell into a new one). Instead, one is using a second, distinct magical effect to provide support to the (unchanged) original spell, allowing it to produce effects beyong its normal duration. "Sustain [the spell]" seems to me the most accurate way of concisely expressing this concept.
Honestly, it seems to me that a lot of people like, for balance reasons, the idea that "Sustain a spell" only applies to Conc Duration spells, and thus they are going out of their way to interpret the guideline so as to have it support that idea. But while I like the idea, I really, really do not think that the actual RAW support it. I think I have outlined out my reasons in sufficient detail that additional posts on my part are unlikely to provide any more information, so I am bailing out of this thread.
Thank you. Yeah, spirits don't follow hermetic guidelines. Although not absolute, I think this example helps to form an opinion.
I'm one of those Storyguides. My gut tells me this spell is pretty abusable, causes a lot of potential "BWUH?" situations at the table, and feels like an end run around semi-permanence in a number of cases.
My cheese-dar is ringing very strongly, and I am not sure i want to let this effect into my saga.
OH, and I'm queeriously's SG, btw.
So, on the one hand, as one of the few players to dip his hands into the rules and spell design, I love his creativity.
On the other hand, this combination of guidelines, Form, and so on makes me concerned that it might become potentially ridiculous. Usable on Spontaneous Magic, so if you can squeak out a momentary or diameter effect, it's "effectively" Permanent?
I have a gut feeling that this spell as is would change how a lot of the Order operates, and in some situations enchantments would be skipped in favor of a Sustaining Circle.
I suppose one approach is to interpret "Circle permance" as sliding from "The Circle's physical made up must be broken" to "Anything crossing the circle - a hair, a feather, a foot - breaks the spell even if the physical circle is not sundered".
Regardless, I'm worried, but also worried about being too much of a hard ass.
Any thoughts, advice, or considerations?
I do sort of agree with the ReVi/MuVi issues regarding Sustained having a particular meaning, if only to keep those two combinations of Arts differentish.
Vrylakos
Talk with your Playres, and make it clear that this is not to be used as a cheap replacement for Duration:Moon or (semi-)permanently enchanted items. If they understand that it is only "long-ish" term, and that "breaking the circle" is easier to do than breaking a steel hoop (or whatever), and so it may have to be recast (ir)regularly, then everyone is on the same page and it should be good to go.