A flared outlet also uses a nut and the fitting, but in this case, it's the end of the pipe itself that creates the seal. Once the nut is placed on the pipe, a special tool called a flaring tool is used. It's placed around the diameter of the pipe near the end in a hole appropriate to that size of pipe, with a flared shape that will form the flare. Once it's secured, another tool is put into position that, when screwed down, will deform the pipe, creating a flare or cone at the end. Once the nut is tightened down, the flared end of the pipe is mechanically trapped between the fitting and the nut, making an extremely strong connection that will require the pipe to be torn or broken to let go unless the nut is loosened. 
Flared outlets are generally used for gas and high-pressure lines, where even a little leakage can cause major problems if left unattended.  VMS stocks brass flare fittings in sizes ranging from 1/4” through 15/16” and in all available shapes including unions, changes of direction (bends, elbows), branches (tees), bushings, extensions, plugs, nuts, reducers, couplings etc., allowing for unlimited possibilities of connection.