The base bidding system is done by filling lots of boxes (or none if you are happy with your lot) and the bid is run periodically usually twice a year to fill in blanks in the basing. For example bid one might be 787 LGW so you would put a 1 in that box however you don't want to be based at LGW on the 73 so you might put a number 2 in the 787 MAN box. If after the bid you didn't get either of them and you would prefer to stay on your current fleet at your current base then the rest of the form would be left blank.
The bidding team sit down and chews over where the spaces are and fill them with the volunteers done solely on seniority order.
It's done in strict order which has some benefits however the obvious draw back is that there have been cases where someone has been waiting for a base (EXT and BOH or example) which have been full for years but when a slot comes up, a more senior person could put a bid in the day before the bid and get the slot.
This is pretty rare though and my last bid had 28 options as I was looking for a command and I got my first preference. In fact the vast majority got option 1.
Clear as mud?