"LINE IN" is a universal audio input - a few mV. You can connect to it, for example, a tape recorder, cd-palyer, mp3-player or anything that sends an audio signal; I just have an additional audio input in the turret.
You CANNOT connect the signal that is intended for the loudspeakers - e.g. from another radio!
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Line out? Is Line in? Because in the subject you ask about "out" and in the post you write "what is it for." entry ....: /
Line Out is a normal output to an external amplifier (or a tape recorder input).