In NM it is a separate classification per the state law as is LP gas. So plumbers or mechanical can be classified. we have to check licenses when they pull permits.
Here what our state has:
B. License classifications. Residential and commercial.
(1) MM-1. Plumbing. Requires four years experience. Install, alter, repair and service plumbing fixtures, and piping, including pneumatic or electric controls and control wiring not greater than 24 volts, concrete supports, and excavating, trenching and backfilling. Includes hot water heating systems not exceeding 30 p.s.i. or 400,00 b.t.u./hour input; piping for fuel, oil and gasoline and for solar energy systems; septic tanks, manholes and sewer lines; irrigation sprinkler systems; swimming pools and spas. Does not include installation of natural gas fired appliances or natural gas piping.
(2) MM-2 Natural gas fitting. Requires four years experience. Install, alter, repair and service natural gas piping and fittings and incidental controls and control wiring, pneumatic control systems, excavating, trenching and backfilling. Includes installation of hot water systems exceeding 30 p.s.i. or 400,000 b.t.u./hour input; steam and hot water boilers; and warm air heating systems such as chimney connections, flues, refractories, burners, fittings valves, thermal insulation, accessories and incidental piping; warm air appliances and other listed gas appliances. May not install LP Gas systems.
(3) MM-3 Heating, ventilation & air conditioning (HVAC). Requires four years experience. Install, alter, repair and service HVAC air handling and refrigeration equipment and piping, including fans, coils, condensing units, self-contained packaged air conditioning or heating units, evaporative cooling units, solar energy systems, ductwork and pneumatic tube systems. May connect water to existing valved outlets, and install controls, and control wiring not to exceed 24 volts. May bid and contract for structural alterations, painting, electrical wiring and other work incidental to this scope of work, provided such work is performed by a properly licensed contractor.
(4) MM-4 Heating, cooling and process piping. Requires four years experience. Install, alter, repair and service hydronic heating, cooling and process piping for steam hot water systems of any temperature pressure range, chilled water systems, condensing water systems and process piping systems. Includes pressure vessels, heat exchangers, boilers, refrigeration water chillers, cooling towers, fuel oil tanks and fuel oil piping, and pneumatic or electric controls and control wiring not to exceed 24 volts. Install high pressure and process piping solar energy systems of any temperature or any pressure conveying gas or fluids other than potable water, and pneumatic tube systems.
(5) MM-98 Mechanical. Requires four years experience. Requires licensure in classifications MM-1 through MM-4 and covers all work described in these classifications, as well as work described in the MS-3, MS-6, MS-12 and MS-14.