When an SlaMap is first created (using AST_SLAMAP), it simply performs a unit (null) Mapping. By using AST_SLAADD (repeatedly if necessary), one or more coordinate conversion steps may then be added, which the SlaMap will perform in sequence. This allows multi-step conversions between a variety of celestial coordinate systems to be assembled out of the building blocks provided by SLALIB.
Normally, if an SlaMap's Invert attribute is zero (the default), then its forward transformation is performed by carrying out each of the individual coordinate conversions specified by AST_SLAADD in the order given (i.e. with the most recently added conversion applied last).
This order is reversed if the SlaMap's Invert attribute is non-zero (or if the inverse transformation is requested by any other means) and each individual coordinate conversion is also replaced by its own inverse. This process inverts the overall effect of the SlaMap. In this case, the first conversion to be applied would be the inverse of the one most recently added.
For example, to use the "ADDET" conversion, which takes a single argument EQ, you should consult the documentation for the SLALIB routine SLA_ADDET. This describes the conversion in detail and shows that EQ is the Besselian epoch of the mean equator and equinox. This value should then be supplied to AST_SLAADD in ARGS(1).
In addition the following strings may be supplied for more complex conversions which do not correspond to any one single SLALIB routine (DATE is the Modified Julian Date of the observation, and (OBSX,OBSY,OBZ) are the Heliocentric-Aries-Ecliptic cartesian coordinates, in metres, of the observer):
AST A Library for Handling World Coordinate Systems in Astronomy