Socket Weld flanges were initially developed for use on small-size high-pressure piping. Their static strength is equal to Slip-On flanges, but their fatigue strength 50% greater than double-welded Slip On flanges.

The connection with the pipe is done with 1 fillet weld, at the outside of the flange. But before welding, space must be created between flange or fitting and pipe.

ASME B31.1 1998 127.3 Preparation for Welding (E) Socket Weld Assembly says:

In the assembly of the joint before welding, the pipe or tube shall be inserted into the socket to the maximum depth and then withdrawn approximately 1/16" (1.6 mm) away from contact between the end of the pipe and the shoulder of the socket.

The purpose of the bottoming clearance in a Socket Weld is usually to reduce the residual stress at the root of the weld that could occur during solidification of the weld metal. The image shows you the X measure for the expansion gap.

The disadvantage of this flange is right the gap, that must be made. By corrosive products, and mainly in stainless steel pipe systems, the crack between pipe and flange can give corrosion problems. In some processes, this flange is also not allowed. I am not an expert in this matter, but on the internet, you will find a lot of information about forms of corrosion.

Also for this flange counts, that principle always firstly a pipe must be welded and then just a fitting.