“does it do so in a separate thread automatically” – yes, threading is all taken of care automatically. It all gets executed within NetworkComms.CommsThreadPool, which is a customised, priority based Threadpool. Originally we used the threadpool provided by .net but it wasn’t flexible enough.
A better resource for asking these sort of threading questions is stackoverflow.com. You can use NetworkComms.Net to easily add networking to any unity projects and should you have any questions relating to networking feel free to post back here.