Your first five years are “engagement frozen” meaning, while you can bid for any seat you like, you can only be moved at BA’s discretion, irrespective of your seniority. Once the five years is up, you can bid for any seat you like and you will get it if your seniority allows. Typically, you should be able to achieve RHS Longhaul at that point, where you will then take on a “type freeze” and will have to stay on that fleet for five “bid years” (which usually equates to about four calendar years).
Command bids work the same way. You can bid whenever you like but you need to have suitable flying hours and seniority. This entitles you to the course but you still have to pass it. Shorthaul commands generally need 8-10 years seniority (although it has been significantly more junior recently). Longhaul commands generally need 17-18 years seniority.
All the above obviously depends on vacancies on each fleet at the time.