From a bit of internet searching, appears to be a hub-type slip-on flange, as opposed to a plate-type slip-on flange. Appears to be UK terminology, not US.
Some of our older welders use the term "boss flange" to refer to what others call a "studding outlet" or "pad flange". We weld these flanges directly to the vessel heads and shell. They can be ordered flat or contured to fit the curvature of the welding surface. They can be ordered single drilled for flange bolts on one side only...or...double drilled for mating flanges on both sides. They save space since there is very little projection from the vessel. They are also easier to coat or line the interior bore hole than pipe nozzles. We buy them from Forged Vessel Connections of Houston.
