Currently the most junior pilot on the 787 is no 530ish out of 798, and has been with the Company 10 years. Vacancies on the fleet are only open to pilots at LGW and MAN, so not an option for those who would like to join the fleet but have overriding reasons for remaining at their current base. It is also a fairly major lifestyle decision as the fleet is purely longhaul, not an attractive option for many. The ultimate arbiter is seniority, so if two pilots bid for the same thing and a suitable vacancy arises, then the most senior pilot will be successful. As Doobler says, this means that you could have had a standing bid for say B737 FO at LGW for 2 years when a vacancy arises, but be unsuccessful because a more senior pilot submits the same bid only the day before. Nevertheless, the golden rule is to bid for what you want rather than trying to second guess the situation. People have missed out on Commands because they had not submitted a Left seat bid as they didn't think it likely to be satisfied any time soon. In a nutshell, a senior pilot's bid for a move to 737 FO CWL will probably succeed, whereas a junior FO bidding for a 787 Command at MAN will have a bit of a wait! He would make that his no 1 bid though and follow up with any number of different permutations. The bid form is not one single bid but allows you to put different bids in rank order.