Oh ok yeah that's what I meant. I saw it and I was wondering if I could partcipate. Thanks for the help.Open threads mean exactly that: they're open for anyone to join. The host of any such threads may have certain stipulations/restrictions laid out in advance though (i.e. - no death/PvP etc...).
Ask threads are usually private affairs between the thread participants, but sometimes there can be certain stipulations (i.e. - "open to Sith only" etc...)
Main Threads are generally reserved for major events meant to shape the direction of the story (i.e. - major battles between factions) and usually require sign-ups from members among the participating factions. In the case of the Battle of Coruscant, however, the threads posted are open to all participants as it is a scripted battle with a pre-determined outcome that will have a major impact on the overall story. The Sith will win and the Republic will lose the seat of their economic power at the end of the battle (note the Republic is not losing it's capital world - the home of the Senate - as that is located on Chandrila).