Put your true order of preference on the CAF form.
If you do not get your first choice school (for whatever reason - eg didn't qualify/oversubscribed/out of catchment) then your second choice school becomes your EFFECTIVE FIRST CHOICE school. If you don't get your second choice, your third choice becomes your EFFECTIVE FIRST CHOICE school. And so on.
The school have no idea where you ranked them - they get told by the LEA who they have made offers to.
Well said, kcg! Parents should think long and hard about their choices and put them in the TRUE order of preference, and should not factor in likelihood of getting in, etc.