47 CFR 95.803 - 218-219MHz Service description.
(a) The 218-219 MHz Service is authorized for system licensees to provide communication service to subscribers in a specific service area.
(b) The components of each 218-219 MHz Service system are its administrative apparatus, its response transmitter units (RTUs), and one or more cell transmitter stations (CTSs). RTUs may be used in any location within the service area. CTSs provide service from a fixed point, and certain CTSs must be individually licensed as part of a 218-219 MHz Service system. See § 95.811.
(c) Each 218-219 MHz Service system service area is one of the cellular system service areas as defined by the Commission, unless modified pursuant to § 95.823.