What are the meaning of the two fields passed around $Cat and $C? Beyond the ubbthreads.php do these get used anywhere else? Is $Cat ever not 0? What's it actually used for that $C isn't used for?

Thanks.

Cat is used to restrict the viewing to specific categories - it's used in every script. We don't restrict the viewing here - so it's always 0.

If you change it to 1 you'll always only see the first category in every script. It's an easy wy to totally restrict what they can see.

The $C is used when the user has "categories" set as their Main Index (instead of the ubbthreads.php page).