The values which may be assigned to this attribute, and its default value, depend on the class of Frame and are described in the "Applicability" section below. In general, the AlignSystem attribute will accept any of the values which may be assigned to the System attribute.
The Mapping returned by AST_FINDFRAME or AST_CONVERT will use the coordinate system specified by the AlignSystem attribute as an intermediate coordinate system. The total returned Mapping will first map positions from the first Frame into this intermediate coordinate system, using the attributes of the first Frame. It will then map these positions from the intermediate coordinate system into the second Frame, using the attributes of the second Frame.
AST A Library for Handling World Coordinate Systems in Astronomy