The DSP-223 has several designations that define the audio paths that connect to it:
Line RX Level is the audio signal "received" by the DSP-223 from the console, i.e. console transmit audio.
Line TX Level is the RX radio audio sent to the console by the DSP-223.
Radio RX Level is the RX audio signal coming into the DSP-223 from the radio receiver.
Radio TX Level is the TX audio signal sent from the DSP-223 to the radio transmitter.