This post is for the benefit of intending MIFF attendees who are as confused as I have been and who have purchased festival passes (or are contemplating it). I've made my own enquiries and ascertained the following:
As described on the MIFF website, there are two types of passes (though there are only limited numbers of these available): Festival Passport and Mini Pass.
The Festival Passport costs $330 ($310 concession) and provides entry to all sessions at all venues except Official Opening and Closing night films. This is the best value if you're intending to see 20 or more films (you could see 100 or more films with this pass - you'd go crazy watching that many, but at least you wouldn't go broke getting there).
The Mini Pass costs $130 ($110 concession) and provides entry to any 10 sessions (same exclusions as above), plus 3 daytime sessions (M-F before 6pm).
If you have one of the above passes, you don't have to book for a session. Your pass is electronically scanned when you enter the venue. But - and this is a big but - if the session is sold out and you haven't booked a seat, you won't get in. So you still need to book to guarantee entry.
Booking online opens from 11am this Friday.
If you're booking online, you don't have to print out the details, but MIFF recommends it. I know from past experience that it's good to have this on hand so you don't forget where and when you've booked for. I like to record everything I've seen, so I'll probably book even for sessions I know I don't need to, just so I can print out the session details.
Screen Legends MIFF members do not have to book for any session at the Forum or Regent theatres. Even if a session is booked out, these members can use the reserved seating. But at all other venues, the same thing applies: if a session is sold out, and you haven't made a booking, you won't get a seat.
My advice in summary: if there's something you definitely want to see, book it online and print out the details. If I had a Mini Pass, I'd book and print out all the films I wanted to see so I could keep track of them all (I did this last year). From memory, you book your sessions, one at a time, and when you're ready, you can print out your festival itinerary. It works quite well. If you have a Festival Passport and you're not fussed about recording the films you've seen, I'd recommend booking just for the films you definitely want to see, printing out those details and just turn up for the rest. Good luck!