The letter "R" as a prefix to this system denotes a "relief car". Plate R-106 is used because the 10th precinct has a regular vehicle in maintenance or repair. The letters "S" and "L" stand for Sergeant and Lieutenant, and denote that vehicle assigned to that ranked officer assigned to the following numbered precinct or zone: S-12 is the Sergeant for Precinct-12 and L-1 is the Lieutenant for Zone-1. The letter "T" Denotes a "Training" vehicle assigned to the police academy. The letter "X" stands for "eXtra"; X-Cars are kept at the city fleet management facility and are available to officers for special duty. "R" and "X" cars are, usually, re-numbered "retired" patrol cruisers in good condition.