Gordo,
Scroggs is spot on as usual. To awnser yr question then yes it is a straight line seniority system with the usual checks and balances. Not that long ago some guys got ahead of the system by virtue of having the req 6000 hrs for cmd, but i would be surprised if there are many F/Os that do not have these hours now. In the near future the recruitment people will squirm in their seats when asked at interviews about the cmd time.
I have been here 5 and a bit years, and have 70 F/os ahead of me and 220 behind me, so who knows??
