No it's not. You are bidding for specific trips, types of trip and days off.
The system generates many (hundreds of thousands..) of lines that satisfy your criteria and when those junior to you have bid it whittles down your lines until you have one possibility remaining at which point it locks in.
If one bid group can't be satisfied it then moves further down your bid groups until one can be so it's possible to rank your bids in decreasing importance.
It's not a preference system at all. As you might have guessed, I think it'll be way better.