S.I.S is an acronym for Special Investigation Section.
It is an Los Angeles Police Department's elite tactical detective squad with a straightforward mandate: Track down the City’s most dangerous offenders and take them off the street. S.I.S. are possibly the hardest-working, most dedicated men and women Detectives to wear a badge.
Alonzo: Hey, Mark, where are you transferring to?
Alonzo: Yeah, S.I.S. Detective.
(from 'Training Day')