There are three locations that are constant in value regardless how they are seen by a viewer. One is coincidence—the near or total spatial overlap of the located object and the reference object. If we say ‘Qiú zài zhuōzi shàng (biān)’ (The ball is on the table), one situation is such that the located object ‘qiú’ is either directly on the table or on/in another object that itself has contact with the table. If we say ‘Qiú zài zhuōzi xià (bian)’, the situation is usually such that the ball occupies a position in the space that is coincident to the space extended downward from the lowest part of the table. Contact or coincidence entails spatial overlap. Therefore, it might be argued that the other eight spatial relations all involve coincidence to some extent, because in the cases of the two examples, if the ball is (hanging) above the table, or if it is under the table, it will occupy a space that is coincident to the domain extended over or below the reference object ‘the table’. Since the located object and the reference object occupy a space that is coincident to each other, these expressions represent the relationship of coincidence.
The other two constant locations are interiority and exteriority. This is illustrated by the example ‘Qiú zài xiāngzi lǐbian/wàibian’ (The ball is in/outside the box). These two locations will hold no matter how they are viewed. Suppose the box is turned upside down, turned on its side or twisted, the spatial relation represented by ‘zài lǐbian’ (inside) will not change. The same account applies to the spatial relation of exteriority.
By contrast, other locations represented by the position words like ‘pángbiān’ (beside), ‘zuǒ/yòubiān’ (left/right side), ‘qiánbiān’ (in front of), ‘hòubiān’ (behind, in the back of), and ‘shàng/xiàbiān’ (on, above/under, beneath) are not constant, their values will vary and interpretations will rely on a framework projected by a viewer, that is, how they are viewed by a viewer. So they are called projective locations. Consider ‘Qiú zài xiāngzi pángbiān’ (the ball is next to the box), the denotation of ‘pángbiān’ (beside) depends on the vantage point of a viewer.
 Loar, J. K. (2011). Chinese syntactic grammar: functional and conceptual principles. New York: Peter Lang.