|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 Mode box lets you choose between Layered and Articulation mode. It also allows for customizing the MIDI the controller used for switching articulations (it defaults to 89). This controller appears as 'Articulation' in the track editor's Controller editor.
The names on the left appear in the Articulation controller editor.
External MIDI Instruments can't be used in the Multi Instrument.