A CPX is, in real life, a Command Post Exercise. In TacOs it is a multiplayer Internet game. The only requirements to play are the current version of TacOps and a good internet connection. Dialups usually do not work. One of the more experienced players will plan and host it (like JeffG). IRC channels and such as disseminated as part of the planning. I don't play TacOps by PBEM but several others do.