<p>If you send in your SIR for one UC, do you have to explicitly decline admission to all the other UC's you've been admitted to or can you just not do anything by May 1st and they'll automatically assume you declined their offer?</p>
<p>No, you don't have to tell the other UCs you declined their offer. The fact that you didn't send an SIR to their campus implies your decline of admission.</p>