Cal. Penal Code § 13128

For purposes of the maintenance of criminal records pursuant to Chapter 4 (commencing with Section 653.75) of Title 15, whenever a person is arrested for a public offense committed while in custody in any local detention facility, as defined in Section 6031.4, or any state prison, as defined in Section 4504, the state summary criminal history record shall include the section number of the public offense violated and information related to the “in custody” status of that person.