Somebody can probably explain it a lot better than me but i'll give it a shot. 690 would be the exit diameter of the muzzle end of the choke. A choke with a .685 would be smaller,and "tighter" than the .690, and one with a .695 would be larger, or more "open" than the .690. You can have 2 chokes from different manufacturers that have the same .690 exit diameter but are internally different. One choke may shoot tight with one brand of shell, and the other choke with the same diameter and shell may be more open due to the internal design of the choke. That's why it takes some testing to find a shell and choke that will work together to give the pattern performance that is desired. Hope this helps some. Maybe someone can chime in and explain it better but that's the gist of it.
