A Z coil is nothing special, it's not a type of coil like Clapton etc - it just refers to a coil that is mounted between posts rather than he conventional way of mounting a coil in front of posts. It's only possible in a few attys, like the O-16, Stumpy etc. Another word for it is "center coil."
I'm currently running a Z coil in my O-16 that is 8 wraps of 26g 316L.
Why Z coil? Well, I like them because it allows me to have airflow come in from both sides equidistant from the coil. That's about the only reason I do it. I don't like duals, just singles, but I do like 2 airflow holes open, and don't like it when one is super close to the coil and one farther away.
Hence why I'm a Z-Sloth.