The two items under the kit list are described below.

Mode:
'Off' means no kit is enabled, so you only have the Add, Sub, and Pad sounds in the Instrument Edit window.

'Multi' means all the kit items will sound together regardless of their note ranges.

'Single' means only the lowest numbered item will sound in a given note range. There will be no overlap.

e.g.
Item 3 has Min.k set to 0 and Max.k set to 60
Item 4 has Min.k set to 40 and Max.k set to 127

In single mode, only Item 3 will sound in the note range 0 to 60 and Item 4 will sound in the range 61 to 127.

In multi mode, only Item 3 will sound in the range 0 to 40, both items will sound from 41 to 60, and only Item 4 will sound from 61 to 127.

There is now a new mode called 'Crossfade' described in detail in:
/doc/Part_Kit_Crossfade.txt

Drum mode:
If this is set then microtonal tuning is ignored for this kit, otherwise it could make drum sounds very unpredictable!
