By default, ``north'' in the new offset coordinate system is parallel to north in the original celestial coordinate system. However, the direction of north in the offset system can be controlled by assigning a value to the SkyRefP attribute. This attribute should be assigned the celestial coordinates of a point which is on the zero longitude meridian and which has non-zero latitude.
By default, the position given by the SkyRef attribute is used as the origin of the new longitude/latitude system, but an option exists to use it as the north pole of the system instead. This option is controlled by the SkyRefIs attribute. The choice of value for SkyRefIs depends on what sort of offset coordinate system you want. Setting SkyRefIs to ``Origin'' (the default) produces an offset coordinate system which is approximately Cartesian close to the specified position. Setting SkyRefIs to ``Pole'' produces an offset coordinate system which is approximately Polar close to the specified position.
AST A Library for Handling World Coordinate Systems in Astronomy