socket weld and butt weld

The easiest answer to this is:

"Read your piping material spec"

Another answer to this is:

Socket weld fittings are used to join smaller sizes of pipe, usually 2" (50mm) an under, which require the strength and security of a welded joint.

Pressure temperature ratings for SW fittings are taken to be equivalent to the following pipe wall thickness designations:

Ftg. Class........Sch. No........Weight

3000..................80...................XS
6000..................160..................-
9000..................-...................XXS

I would also suggest reading ALL the articles in the training section, particularly the fittings article ( pipingdesigners.com/Training%20- ... n%201c.htm ).
You may find it useful to print out and keep these articles for reference.