It's merit based, but technically "selective enrollment" is a distinct "tier" of the non-boundaried schools.
"Selective enrollment" covers Brooks, Lane, Jones, Linblom, Young, Payton, Westinghouse, Curie, Northside, and definitely a few others I am forgetting. Admission is 1) grades from gradeschool (I think 7th grade) 2) gradeschool attendance 3) admissions test. The city is split into 4 tiers by zipcode based on average income, and they have to take a certain % from each tier. (I think 25/25/25/25 if I recall correctly)
EDIT: The ones I missed were south shore, king, Hancock, and then Kenwood, Morgan Park, and Taft all have SE while also being a neighborhood school for those in boundary. Curie is not one.
There are a bunch of other schools like Von Stueben, Simeon, etc. that are not technically "selective enrollment", but you still have to apply or do a lottery or something. I am less familiar with these.
Lastly, you can apply to special programs at normal neighborhood schools. So Lincoln Park, Taft, Morgan Park etc. would have kids from outside the area who got in by applying to like the IB program or something.
Not sure which (or all) of these they are proposing phasing out.