This minor group includes maintenance trade contractors and heavy equipment and transport operator contractors who own and operate their own businesses; and supervisors who supervise and co-ordinate the activities of workers in maintenance and heavy equipment and transport operator trades. They are employed by railway transport companies and urban transit systems; utilities; aircraft manufacturing, maintenance, repair and overhaul establishments; airlines and other aircraft operators; industrial machinery and transportation equipment and vehicle manufacturers; companies that own and operate heavy equipment and heavy equipment dealers, rental and service establishments; elevator construction and maintenance companies; refrigeration and air conditioning installation contractors, retail and servicing establishments and food wholesalers; engineering firms; motor vehicle dealers, garages, truck and trailer dealerships, fleet maintenance companies, and service stations; heating systems installation and service companies; appliance repair shops service companies and repair departments of retail and wholesale establishments; and electrical repair shops, service shops of electrical equipment manufacturers and maintenance departments of manufacturing companies; or they may be self-employed.