You state 6 schools in order of preference - say schools ABCDEF, with A as your favourite.
Once all the applications are in, each school looks at the criteria for every single applicant and ranks them in order according to theier admissions criteria. So maybe A is a faith school which only admits non-faith students after siblings and church attenders, and can't offer you a place. B is an oversubscribed school several miles away, C is your second nearest comp, D another faith school, E your third nearest comp and F your closest comp.
Then pretend that:
F tells the LA that they can admit you on distance criteria.
E says they cannot admit - too full, you are too far away.
D offers you a place because this year there has been a lower than average demand from church attenders and you get one of the 'non faith' places on distance.
C does not offer you a place (too many people live closer),
B does not offer a place,
and neither does A - all places filled with church attenders.
The offer that you will receive on National offer day is D. The place that would have been offered from F has been re-allocated to the applicant next down the ranked list - and you will automatically be placed on the waiting lists for schools C, B and A.
Accepting the place at D does not jeopardise your chances of being offered a place from the waiting lists of the schools you rnaked higher than the offered school. Once everyone has sent back theier appcetances, they e-shuffle again - if you were top of the waiting list at C, you will be offered the first place that becomes available, and so on.
There can be a lot of movement over the summer.
Waiting lists are maintained in the same ranked order according to the admissions criteria. There is a date in the summer when late applicants are added into the lists (after the first offers have gone out) and at this stage you can even find yourself going down the waing list - if someone who lilves closer joins the list.
You will be put on the waiting list of all schools listed higher in your preference than the one allocated, and there is a date when you can put yourself on the waiting list for any other school, whether or not you applied in the first place.
If for some reason you sudddenly decided you had made a mistake and you actually preferred school F to school D then you would need to put yourself on the waiting list for school F - you can't backtrack and claim the place you would originally have been offered if D hadn't put you on their list.
AND
If you are not offered a place at an of the schools listed the LA will allocate you in any school that has a place, no matter how far away.
This is why it is important to put your prefernces in the exact order you really prefer them, and to include a school that you have a good chance of getting into.
You cannot 'beat the system' by only listing popular over-subscribed schools that you stand little chance of getting, or only putting down one or two preferences so that 'they have to give you one of those'. No. They don't!