As a public institution, the UC system prioritizes admissions for the state's top students. The Statewide Index identifies California-resident students in the top 9 percent of California high school graduates and offers these students a guaranteed space at a UC campus - though it may not be at a preferred campus or even one they applied to.

 

For more info https://admission.universityofcalifornia.edu/admission-requirements/freshman-requirements/california-residents/statewide-guarantee/#:~:text=The%20Statewide%20Index%20identifies%20California,campus%2C%20if%20space%20is%20available.