OK, here is an expanded Container Targets insert. Comments?
Container Targets
Spells with "container" Targets, including Circle, Room, Structure, and Boundary, can work in one of two ways.
First, they can affect any valid target within the Target container at the time of casting, and continue to affect those targets even if they leave the original Target area, for as long as the spell lasts. It does not affect anything that enters the Target container later, even if the spell is still in effect.
Second, they can affect any valid target within the Target container during the spell's period of effect. In this case, a valid target that leaves the container ceases to be affected by the spell, and a valid target that enters (or re-enters) the container is affected, until it leaves or the spell expires.
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 put pink dots on people's foreheads with Target: Room and Duration: Moon could work in two ways. In the version that works in the first way, everyone in the room (on whom the spell Penetrates) at the time of casting gets a pink dot on their forehead, and this pink dot remains on their forehead until the new moon and full moon have both set. In the version that works in the second way, anyone who is inside the room (on whom the spell Penetrates) until the new moon and full moon have both set gets a pink dot on their forehead, even if they were not in the room when the spell was cast. When they leave the room, the pink dot disappears, although it reappears if they go back into the room. 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.
If the container ceases to exist as a container before the spell duration ends, the effect depends on the type of spell. If it is of the second type, the spell ends when the container ceases to exist. For example, removing the roof and one wall of a Room would mean that there was no longer a Room, as would placing a wall down the middle of the room. If it is of the first type, the spell is unaffected, unless the Target is Circle, in which case it ends when the circle is broken.
Most containers do not normally move, but they can. A Structure, for example, can be a ship. This is irrelevant to spells of the first type, which affect anything within the container at the time of casting, no matter what happens after that. Spells of the second type move with the container. Note that wards, including Aegis of the Hearth, are all of the first type, and target the interior of the container at the time of casting. If the container moves, part of it moves outside the ward. (Note also that, in Mythic Europe, the earth neither spins nor moves through space: things that do not move relative to the earth do not move at all.)