The way to find this out is to think about when the attribute is important, and see it in use. In this case, the attribute is used in the GET command, which you can find if you do Filter - Show Library Elements. In fact the command is called TAKE, and in turn uses DoTake. Looking at the code, turns out the volume is calculated each time, using this code:
volume = 0
foreach (obj, GetAllChildObjects(game.pov)) {
if (HasInt(obj, "volume")) {
volume = volume + obj.volume
}
}