Mixcraft has no Instrument Definition files. It uses left and right click menus from an Instrument track's keyboard button to set up your voices.
 
It is a bit more flexible than SO3 and has a lot of options including key range, velocity, and assigning effects.
 
Once its all setup you can save it as a Preset for future use.
 
Like SO3 it can also use PRV Program Change controllers to set and/or change the voice. Unlike SO3, it DOES recognize my SQ1's panel Bank selection. So here I have 2 options to trigger sounds:
1. Set the SQ1's Bank to either Internal or ROM and select the patch # in Mixcraft, or
2. Set the Bank and patch #s all in Mixcraft.
 
IOWs, Mixcraft does NOT require the PRV controller when selecting the SQ1's panel Bank from the synth.
 
Like SO3 the PRV controllers can be used to change a track's voice several times throughout the track.
 
Ref: Mixcraft 8 manual pg. 271-283.