I think they can do just about anything you want, but the most common options are the leather (roughout or smoothout), the tooling, the seat, the rigging, etc.
I'm not sure if all their options are available on all their saddles though. I've never seen one of their barrel saddles with a drop plate rigging...though I'm sure they could do it.
What I found helped me the most was to browse around different saddles and decide exactly what I wanted. I found one thing on one saddle that I really liked and another thing on another saddle that I really liked. When I called them, I just told them what I wanted.
You can browse their website and get an idea of the most common options available.