When an SlaMap is first created, it simply performs a unit (null) Mapping on a pair of coordinates. Using the AST_SLAADD routine, 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 AST_SLAADD routine.
AST A Library for Handling World Coordinate Systems in Astronomy