What the difference between weldolet and thredolet?

A weldolet is a fitting you weld on a (run) pipe, and then butt weld a branch pipe to it.

A tredolet is a fitting you weld on a (run) pipe, and then screw a threaded branch pipe to it. Some people call this a threadolet

A sockolet is a fitting you weld on a (run) pipe, and then socket weld a branch pipe to it.

A nipolet is the same as a weldolet or thredolet but has an elongated branch connection. You have to specify if it's a welded or threaded nipolet.