2 things the diameter of the shaft and the head type .. well 3 if you include the length

I still like the old Campag 2 bolt heads at the top Campag seatposts , but they were designed to use leather saddles ..

The side clamping Race Face is an interesting design there is 1 bolt squeezing the rails , and a second one gripping the seat post shaft

you can change the saddle angle with it and not loosen the bolt gripping the saddle rails..