The RAD/INT switch:
RAD means you transmit through the channel for which the MICrophone button is selected - this can be any radio, the interphone, the Public Address system, etcetera.
Selecting RAD is the same as keying the Push To Talk switch on the hand mike, or on the base of the sidestick.
The RAD/INT switch is springloaded from RAD to the neutral position.
The INT position is a fixed position, meaning, the switch will not jump back to neutral when released. The INT position "transmits" over the interphone (only) so you will have a hot mike for conversation with other cockpit crew and with ground engineer. Using this hot mike, you can leave the MIC button selected for the radio that you use for contact with ATC.