Simplex
Simplex is a method of communicating using only one frequency.
866.0125 frequency can be used to talk mobile to mobile, base to base, mobile to base, etc. (without going through a repeater).
Typically, such transmissions are limited in the distance over which they can be heard, since they are dependent on the frequency, output (in watts) and antenna height.
BTW,
Semi-duplex is a method of communicating using two frequencies, where each side can talk but one at a time. (Our state patrol uses 154.680 for the bases and 154.800 for the mobiles).
Full-duplex is both sides talk over each other, etc. (Your telephone)
Repeater = input frequency is "repeated" at a higher output (and usually on a taller antenna) on a second output frequency. May be combined with other repeaters, hardware and software to create trunked systems which we can love or hate (depends on IF encryption is used)javascript:emoticon('

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Very Happy