Yes the available bits field controls mission availability.
A simple setup for mission availability and order control might look like this:
Available bits: !91 (makes sure this bit is clear to make the mission available)
On Success : b91 & !b92 (set bit 91{so you can't get it again} and clear bit 92{to make it available})
The next mission would be like this:
Available bits: !b92 (makes sure bit 92 is clear)
On Success : b92 & !b93 (sets bit 92 and clears bit 93)
And so on and on and on...
It can get much more complicated than this though. This is about as simple it can get.
I did use unused bits for the above example.
(edit) The Nova Bible (in the documentation folder) explains the meanings of the symbols I used above and others that are used for different effects. (/edit)
This post has been edited by Ant'Iglias II : 25 November 2007 - 05:47 PM