This SWI sets the current printer dumper by calling PDumper_Reason_SetDriver. The *Command in R2 is issued if the requested dumper is not currently loaded. The blocks in R3 and R4 are both copied by PDriverDP - hence the originals may be discarded. The R3 block contains the name of the palette file to use - this gets passed to the dumper module now, but is not really needed until PDumper_StartJob, which is usually called from PDumper_Reason_StartJob.
See PRM 3, page 656.