Section 308.4 state...."This group (I-3) shall include, but not be limited to, the following..."
The holding cells meet the intent of I-3 occupancy..."a portion of a building (i.e. the police station) that are inhabited (not defined as to period) by one or more persons who are under restraint."
CBC 308.4 Group 1-3.
one or more persons who are under restraint.