The Fender Mustang III/IV/V include an Effects Loop to connect external effects devices to your amplifier.
The Fender Mustang III amplifier only accepts mono effects devices as it has a mono Effects Loop. "Y" adaptor cables can be used to connect a stereo device into the Mustang III's mono effects loop.
The Fender Mustang IV and V amplifiers can accept stereo or mono effects devices at line level. In use with the Mustang IV and V, a mono-in, mono-out effect unit will use one send and one return, and only effect one side of the stereo image when used without appropriate adaptors. "Y" adapter cables can also be used to connect a mono effect to the Effects Loop on the Mustang IV and V amps so that both left and right channels of the Effects Loop will be connected. However, the resulting audio signal be dual monaural and not stereo.
Connect the amp's FX SEND jack(s) to the input(s) on your external effects device and connect the amp's FX RETURN jack(s) to the output(s) on your effects device. Additionally, you can use the Effects Send as a Line Output for recording puposes or a live sound application (Mustang III-V/V1 Series).
The Mustang Floor has a mono Effects Loop that is fixed in the "pre amplifier" position.