planar curved
If for analytical purposes objects can be treated as residing on a plane, or (non-curved) three dimensional space, then the first choice is appropriate. If the object or objects in question are large enough, or if the distances between them are large enough, it may be necessary to consider them as resident on (or with reference to) a curved space, i.e., the surface of the earth's ellipsoid, in which case the second choice is given. This distinction is important for example, if interpolation is required between points on an arc. This is used as a domain in CoordinateSystem.