If SkyRefIs is set to "Origin", then the coordinate system represented by the SkyFrame is modified to put the origin of longitude and latitude at the position specified by the SkyRef attribute.
If SkyRefIs is set to "Pole", then the coordinate system represented by the SkyFrame is modified to put the north pole at the position specified by the SkyRef attribute.
If SkyRefIs is set to "Ignored" (the default), then any value set for the SkyRef attribute is ignored, and the SkyFrame represents the coordinate system specified by the System attribute directly without any rotation.
AST A Library for Handling World Coordinate Systems in Astronomy