Here is information in the UCB thread about LOR’s which can answer your question:
Now, all UC campuses can ask for LORs as part of “augmented review,” but can refer no more than 15% of applicants for augmented review. This is a contrast to the 30% who were asked for letters by Berkeley last year:
http://www.mercurynews.com/2017/07/17/the-uc-application-process-is-about-to-change-and-some-people-dont-like-it/
http://www.latimes.com/local/lanow/la-me-uc-regents-recommend-letters-20170712-story.html
http://admission.universityofcalifornia.edu/counselors/files/augmented-review-policy-2017.pdf
Students can also be asked for fall semester senior year grades, and can be given a questionaire as part of augmented review.
Here are the five specific conditions under which an applicant might be selected
for an augmented/supplemental review (quoted from the augmented review policy linked above):
Evidence of focus on an area of special talent which may have limited a student’s
time to participate in a broader range of activities.
Evidence of character traits that imply a strong likelihood of making a significant
contribution to campus life.
Evidence of significant academic achievement or the potential for academic
achievement at the University in spite of extraordinary or compound
disadvantage or learning difference, or physical disability or other unusual
circumstances.
Evidence of significant improvement in the academic record accompanied by one
or both of the following: reasons for the initial poor performance; and/or sustained
and in-depth participation in educational outreach programs, which demonstrate
the applicant’s commitment to succeed academically within a challenging
environment.
Evidence of relative lack of access to, counseling about, or support to take
college preparatory, honors, Advanced Placement (AP) or International
Baccalaureate (IB) classes or required college entrance examinations.