Use for atomic clocks - not any other type of clock.
Atomic clocks are the most accurate time and frequency standards known. They are used as the primary standards for international time distribution services, to control the wave frequency of television broadcasts, and in systems like GPS.
An atomic clock uses an electronic transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its time-keeping element.
There is more information about atomic clocks here.
