These projection parameters correspond exactly to the values stored using the FITS-WCS keywords "PV1_1", "PV1_2", etc. This means that projection parameters which correspond to angles must be given in degrees (despite the fact that the angular coordinates and other attributes used by a WcsMap are in radians).
The set of projection parameters used by a WcsMap depends on the type of projection, which is determined by its WcsType parameter. Most projections either do not require projection parameters, or use parameters 1 and 2 associated with the latitude axis. You should consult the FITS-WCS paper for details.
Some projection parameters have default values (as defined in the FITS-WCS paper) which apply if no explicit value is given. You may omit setting a value for these "optional" parameters and the default will apply. Some projection parameters, however, have no default and a value must be explicitly supplied. This is most conveniently done using the "options" argument of astWcsMap (q.v.) when a WcsMap is first created. An error will result when a WcsMap is used to transform coordinates if any of its required projection parameters has not been set and lacks a default value.
A "get" operation for a parameter which has not been assigned a value will return the default value defined in the FITS-WCS paper, or AST__BAD if the paper indicates that the parameter has no default. A default value of zero is returned for parameters which are not accessed by the projection.
Note, the FITS-WCS paper reserves parameters 1 and 2 on the longitude axis to hold the native longitude and latitude of the fiducial point of the projection, in degrees. The default values for these parameters are determined by the projection type. The AST-specific TPN projection does not use this convention - all projection parameters for both axes are used to represent polynomical correction terms, and the native longitude and latitude at the fiducial point may not be changed from the default values of zero and 90 degrees.
AST A Library for Handling World Coordinate Systems in Astronomy