I think there is an element of opt-in or opt-out (of testing) too? I think an area with selective schooling generally operates an opt-out policy and takes about a quarter to a third of pupils into grammar school. There is some element of catchment area with all passes securing a place.
In a superselective, an opt-in system of testing operates, and no catchment, so children from other counties or even other countries may take the test and be offered a place. A pass does not guarantee a place - the highest ones get in, with different schools able to set different scores. I think generally there are far fewer children in an area going to grammar school in areas with system (around 4% here in Gloucestershire, vs about 28% in Bucks which is selective).