There are two kinds of forum RP in SWC. The first is the Meeting Hall, on the RPG Centre, which is generally free form with the plots and results of actions being determined by the participants (some with more of a designated plot than others).
The second is, of course, the White Scenarios, which have a definite plot that is being controlled by a Games Master, who also determines the outcome of actions (so instead of shooting and killing 5 stormtroopers, you shoot at them and he tells you if/how many you hit). Each scenario typically has sign-ups in the beginning, with the GM then choosing participants. Moderators from the WS team then review the posts made, and award XP to participants based on how well they write/RP (and there are various guides and guidelines on the WS boards as to how they determine that).
The WS board does require a separate registration/login, as it's been hosted off of the main SWC site for quite some time, for various reasons.
"May the Grace of Ara go with you, and His Vengeance be wrought upon your enemies."