I've had a go at writing up the general version of static and dynamic targets, and it seems to require a particular answer to the question about growing targets as well. What do people think?
Static and Dynamic Targets
It is possible for most spell targets to change while a spell is active. For example, an Individual person includes that person’s hat, but they can take the hat off and put a new one on. The hat they removed is no longer part of the Individual with the person, and the hat they put on now is. Spells can work in one of two ways.
First, they can affect any part of the Target at the time of casting, and continue to affect those things even if they cease to be part of the target. These are referred to as “static targets”, because the target does not change while the spell is in effect.
Second, they can affect any thing that is part of the Target during the spell's period of effect. In this case, things that cease to be part of the target cease to be affected by the spell, and something that becomes part of the target is affected, until it leaves or the spell expires. These are referred to as “dynamic targets”, because the target may change while the spell is in effect.
The way that a particular spell works is fixed when it is designed, and cannot be changed by the casting magus, although a spell working in one way is similar to a spell that is identical apart from working in the other, and so knowledge of one gives a bonus to inventing the other.
For example, a spell to make someone invisible, with Individual Target, can work either way. With the static version, if the target removes her hat and puts it down, the hat remains invisible. If she puts a different hat on, that hat remains visible, and appears to be floating in the air. With the dynamic version, if she removes her hat, it becomes visible when she puts it down, and a different hat becomes invisible when she picks it up. (The spell would cover small carried items, so she does not need to put the hat on.) These two versions are two different spells, and a maga who wants to cast both (and cannot reliably cast them spontaneously) needs to create two spells.
Static spells continue to affect the original target for their duration, even if the target changes so that it would no longer be a valid target for the spell. In the example above, the hat and person no longer constitute an Individual, but the spell remains in effect on both. Similarly, a static spell cast on a Group of grogs would continue to affect all the grogs even if they mingled with a crowd.
Dynamic spells fail if the target of the spell ceases to qualify as a target of the spell. The invisibility spell in the example continues to work because the Individual target does not cease to exist because she changes hats. However, if a supernatural effect made her grow too big to be affected by the initial spell, it would fail. Similarly, a dynamic spell cast on a Group of grogs would fail completely if they mingled with a crowd, because they would no longer form a Group.
In some cases, it may not be clear whether a target has changed or ceased to exist. Troupes should decide these on a case-by-case basis.