According to the NM CCW regs, it is based on caliber:
NM CCW regs
The section that covers caliber is 12-19-6, subsection C-4:
"The category and largest caliber of handgun that the licensee is licensed to carry, with a statement that the licensee is licensed to carry smaller caliber handguns but shall carry only on concealed handgun at any given time."
It is my understanding that "Category" relates to whether the firearm is a revolver or semiauto, so if you wanted to be able to carry either one, you would need to qualify with both types of sidearms...again, most CCW instructors should have both types on hand for you to qualify with, so you are covered on either one.....