The show focuses on the Los Angeles County Sheriff's Department. While they do have the ability to make arrests anywhere in the county, they generally do not patrol in the city of Los Angeles (as represented in the story) since it's covered by L.A.P.D. instead. The Sheriff's Department works in unincorporated towns like Altadena and East Los Angeles, incorporated cities that contract with them like Santa Clarita and West Hollywood, and transit agencies like Metrolink and Los Angeles MTA.