If I SIR to UCSC does that mean I can't accept UCI or UCSD if I get accepted off their waitlist?

Colleges and Universities A-Z University of California - Santa Cruz
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No.
You need to SIR to UCSC by May 1. If you are accepted off the waitlist from UCI or UCSD, then you can withdraw your SIR from UCSC and accept your waitlisted school’s acceptance. You will however lose your enrollment fee from UCSC.