If a Symbol value has not been set for a Frame axis, then a suitable default is supplied.
The default Symbol value supplied for simple Frames is the string `` Domain n '', where n is 1, 2, etc. for successive axes, and Domain is the value of the Frame's Domain attribute (truncated if necessary so that the final string does not exceed 15 characters). If no Domain value has been set, "x" is used as the Domain value in constructing this default string.
Specialised Frames (SkyFrame, SpecFrame,etc.) re-define the default Symbol value to be appropriate for the particular co-ordinate system being represented.
KAPPA --- Kernel Application Package