"intermediate world coordinates
"(IWC). These Frames will have Domain name
"for primary axis descriptions, and
"for secondary axis descriptions, where
"is replaced by the single alternate axis description character, as used in the FITS-WCS header. The default value for
FITS-WCS paper 1 defines IWC as a Cartesian coordinate system with one axis for each WCS axis, and is the coordinate system produced by the rotation matrix (represented by FITS keyword PCi_j, CDi_j, etc). For instance, for a 2-D FITS-WCS header describing projected celestial longitude and latitude, the intermediate world coordinates represent offsets in degrees from the reference point within the plane of projection.