DigiTech VOCALIST VL3D Owner's Manual page 17

VL3D connected to a MIDI Interface
When the VL3D is not connected to a computer using the USB port, all MIDI
data sent to the MIDI In of the VL3D is passed on to the MIDI Thru jack. This
allows you to chain other MIDI devices, such as sound modules, to the VL3D.
13