There are four types of schools in the Chicago Public School system: neighborhood schools, selective schools, magnet schools and charter schools.
Neighborhood schools in Chicago must enroll any student who lives within their boundary. Every student has a neighborhood school they can go to. Tiers don't matter for enrollment.
Selective schools, which admit students from across the city, are different. Students must apply to them, and no one is guaranteed a seat because of where they live. Most of the seats are filled through the tier system. Selective schools and programs include:
Selective schools in Chicago have quotas. In Chicago, a student's chance of getting into the city's top, selective schools depends on where he or she lives. That's because selective schools use admissions quotas: Chicago Public Schools requires the schools to reserve an equal number of spots for students coming from poor and wealthy neighborhoods.
If schools didn't do this, students from well-off places would be admitted in disproportionate numbers, because they tend to have better admissions scores.
Rightly or wrongly, the quota tries to keep wealthier students from dominating selective schools.
The result: a high-achieving student from an impoverished area has a better chance of getting into a selective school than a similar student from a richer area.
Info from: http://cpstiers.opencityapps.org/about.html