ASME B16.25 covers butt joints.
AWS D10.X, where X is placeholder and depends on the type of pipe, i.e., carbon steel, austenitic stainless, aluminum, etc.
Sockets, slip-on flanges, and integrally reinforced branch fittings ("weld-O-Lets, Thread-O-Lets", and other brand named fittings) are specified in the particular pressure piping code. Fit-up requirements can usually be found in the product catalog that is usually on the manufacturer's web site.
Al
Best regards - Al