|The Multi Instrument turns up to 5 MIDI instruments into a single instrument. You can either use a MIDI controller to pick one of the sounds (see articulations), or you can play them all at once (aka 'layering').|
There's a full mixer with effect slots to mix the instruments.
Multi Instrument window
The Controller box defines the controller used for switching articulations. This controller appears as 'Articulation' in the track editor's Controller editor. Choose 'None' if you want to layer all instruments.
The names on the left appear in the controller editor.
External MIDI Instruments can't be used in the Multi Instrument.