What are the parameters that can be used by the remapping code?

When I tried to remap the code, I found that there are many letters that cannot be defined as parameters, and cnc will report an error. Unlike what I saw on the official website, the following is the code for the test error: linuxcnc.org/docs/devel/html/remap/remap...he_argspec_parameter
Including a/b/c/u/v/w/o/n/g/m:
               

Most letters are used in gcode or axes names, so for free use maybe there is only R L Q P left, as in the boring cycles.