Spugeddy is really close with his response. With menu 7 TDR on, the radio will "listen" on both A & B frequencies. If Menu 34 TX is set to "both", when you push the PTT button, the radio will transmit on the last channel that had a receive signal, regardless of the which one is actively selected. example: TDR is on, you are selected on channel B (the lower one), and the radio receives on Channel A, the next time you push the PTT button, the radio will transmit on Channel A.
If you set the Menu 34 TX to A, the radio will only transmit on A, B will only transmit on B. If you set this menu to off, the radio will transmit on whatever channel is selected, as indicated by the arrow