You can’t go on a waiting list at this stage in y6 because the applications aren’t in, places haven’t been allocated and there is no waiting list yet.
Your catchment school will take people in the order that they meet the criteria, it doesn’t matter if they are first choices or not.
Just say you apply for schools A, B and C and each school has 200 places.
School A is very popular and you are low down the criteria for a place. Your child is 500 on the list and isn’t offered a place.
School B is also popular, but you are higher up the list (eg on distance). Your child is 250th.
School C is your catchment school. Your dc is high up in the criteria, 50th on the list. You have put it third but you still get the place. There will be people lower down the list who have put it first.
However, when they look at the people who applied to school B, 50 dc who are ahead of you on the list have actually put school A or school C as their first choice and are in the top 200 so are offered a place there instead so your child shoots up the list and is offered a place at school B. The place that your child could have got at school C goes to a child another child.
popular schools do “fill up with first choice” dc in the sense that if one school is really unpopular and is undersubscribed then whoever puts it as their first choice will be given it. It’s not so much “you have to put this as first to get a place” it’s “if you put set on fire school first then you won’t get an offer from us because you will get one from fire school”. If a school has 1000 applicants for 200 places and another school has 180 applicants for 200 places then everyone who has applied for the second school as their first choice will be allocated it, even if they were in the top 200 for the over subscribed school.