Also note that "machinegun" is the legal term, used in Title II of GCA 68, aka the NFA.
The term full automatic is the same as machinegun. The term selective fire means a firearm that can be fired semi-auto or full-auto; legally, a selective fire firearm is a machinegun.
Some submachineguns, a term usually defined as a pistol caliber machinegun, are selective fire, some are full auto only. Most full size machineguns are belt fed and are full auto only, though there are exceptions, including the .50 M2, which has a semi-auto capability (and is surprisingly accurate in that mode).