I put asterisks next to the ones I have checked.
Default Follow Options
Choose which types of post should automatically add a thread to your "followed threads" list:
* Direct responses to your posts
*Threads that you start
* Threads that you reply to
*Threads that are started as friends-only
Threads that are replied to as friends-only
Threads that your friends start
Threads that your friends reply to
Child (related) threads of threads you follow