[CF-metadata] Question about cf conventions-dimensionless coordinates

[Hand, Brian]

I'm trying to find out some general information about cf 1.3/1.4.  For the standard_name land_ice_sigma_coordinate, I wanted to know if there is a formula definition?  I've run it through the compliance checker to tell me that there is no formula for the formula_terms specified.  I guess I'm a little bit unclear on the dimensionless coordinates in general.  It says only the ones under Appendix D? have formula definitions?  Are some user defined? 
 
Thanks,
Brian.