Applied to a room, it is invoked early in the processing cycle, before the room's description is printed.
Nothing happens for objects on supporters or in containers.
The last of the descriptive properties, describe can contain a string, but really requires a routine. It's a bit of a specialized property, not one for which a natural example is easy to contrive. If you think about it, the label object doesn't contribute much to the game. That is, the bottle is a real object, so we try to give it a convincing description. Unfortunately, in doing so we're likely to imply the existence of sub-objects, in which the player may well take an interest.