Stucano - you have to be on a permanent contract and have the minimum hours to apply for the command process. You do not have to hold a particular rank, although in practice you are more likely to be an SFO, but that is not a stated requirement. There are then a significant number of hoops to jump through before you become a Captain. None of them are based on who your mates are, your colour, race, beliefs, nationality or anything else. You need to be very competent and a credible future Captain - nothing more and nothing less. It is not therefore automatic, but we have we have more promotions per year than any other airline in the world that I know of. Every last one of them go on to the standard Captain's deal, despite the constant warnings from the likes of Narrow Runway. The only variation to that is those guys that take up the Lisbon deal, which is not very good but offers the advantage of getting a command sooner than might otherwise have been the case - horses for courses etc.
For practical purposes we run a seniority system for command, although the word 'seniority' is banned from our vocabulary! The seniority system comes into play when you register your name for pre-command and get the appropriate interview. At that point you are frozen on a list and the date your name was placed on that list determines the order in which you will be offered a command course subsequently, subject to successful completion of the assessment process. It is about as fair as you could hope for and the vast majority of people are successful. The downside is that you have to go where the command vacancies are - that is more often than not Gatwick. If you have a wife and kids in Milan or wherever, that can be a problem. If you are waiting to get back to a popular base, you may wait a long time as vacancies only rarely occur. To change base you just put your name on the Transfer List, and when your name comes up you can accept or decline the offer. As you might expect, tensions can arise and there are not always easy answers. For example, when the Madrid base closed the guys there got put ahead of all the transfer listed people to move to bases of their choice - a great deal if you are in Madrid, but pretty grim if you were Number 1 on the list and found 10 guys now in front of you taking up the only free slots that had appeared for years! This is a vast subject that is largely outside the scope of this thread, but I hope that helps. No system is perfect but it is very transparent, and no more can you ask for.