When an SlaMap is first created, it simply performs a unit (null) Mapping on a pair of coordinates. Using the astSlaAdd function, a series of coordinate conversion steps may then be added, selected from those provided by the SLALIB Positional Astronomy Library (Starlink User Note SUN/67). This allows multi-step conversions between a variety of celestial coordinate systems to be assembled out of the building blocks provided by SLALIB.
For details of the individual coordinate conversions available, see the description of the astSlaAdd function.
AST A Library for Handling World Coordinate Systems in Astronomy