A socket welding flange has a similar shape to a slip-on flange, except that it has a bore and a counter bore. The two different bores create a socket which holds the pipe in place before welding.
The diameter of the smaller bore is the same as the inner diameter of the matching pipe. Once the pipe is inserted and lands on the inner socket, the outside of the flange and pipe is welded.
Since the pipe and flange meet exactly on the socket, this structure brings less frictional restriction in flow on the connection.