Repeater vs. simplex: what is the difference?

Simplex or direct operation

With simplex operation, one radio transmits directly to another. It is simple and useful for local communications, but coverage depends heavily on antenna height, terrain, buildings, radio power and the surrounding RF environment.

Repeater operation

A repeater listens on an input frequency and retransmits the audio or digital signal on an output frequency. Because the repeater antenna is often located higher and in a better RF position, portable and mobile radios can communicate over a larger area.

Why systems often include both

A repeater channel can provide normal wide-area communications while a direct channel gives users a local option if they are close together or if the repeater is unavailable.