A cap may have a solvent weld socket end or a female threaded end. The other end is closed off. A coupling connects two sections of pipe together. Couplings may have solvent weld socket ends or female threaded ends. A cross connects four pipe sections together.
Crosses are special order parts at many suppliers.
Crosses create a great deal of stress on the pipe because they have four connection points. I recommend that you avoid using crosses in most situations. Use two tees. Female adapters are used to add a female threaded pipe connection on a solvent welded pipe. Never use female adapters when converting to a metallic pipe.