A tridimensional area of some object is some kind of hole in that object, which has a particular morphological saliency. Probably no saliency rule can be stated; for example, body spaces are 3d areas which are considered parts of an anatomical structure, although the most you can say structurally is that they are surrounded by anatomical-structures. In other words, a 3-d area is a mereologically relevant hole in some object.
(<=> (3d-Area-Of ?A ?B) (And (Hole-In ?A ?B) (Part-Of ?A ?B)))