A two- or three-dimensional coordinate system that consists of any combination of coordinate axes
not covered by any other Coordinate System type. An example is a multilinear coordinate system
which contains one coordinate axis that may have any 1-D shape which has no intersections with
itself. This non-straight axis is supplemented by one or two straight axes to complete a 2 or 3
dimensional coordinate system. The non-straight axis is typically incrementally straight or
curved. A UserDefinedCS shall have two or three axis associations.