Answer: This type of dual connection is pretty common these days on tub/shower valves and the like. As you say, if you are using 1/2" copper it will slide into the fitting and is sweat-soldered in place. Iron pipe is not/should not be used anymore for water, but if you are using 3/4 copper, or pex or cpvc or similar, then you use a female adaptor on the pipe and thread it onto the valve. Personally, I prefer the soldered connection, but a properly made threaded connection is probably just as good. Remember to remove the plastic valve cartridge from the valve body before you do any soldering. And it's a good idea to flush out the piping before you reinstall the cartridge by turning on the water with the cartridge out (be prepared, water will shoot out everywhere) to get rid off any loose bits of solder or copper or just dirt in the pipes; otherwise it will just plug up the ports in the valve later.