Thanks and a question about SF2 chaining

Thanks for your positive comment, I'm glad you like my work.

VirtualMIDISynth uses BASSMIDI.dll (from un4seen.com) as MIDI rendering engine, so refer to it for the specs.
BASSMIDIDRV is a driver similar to VirtualMIDISynth and uses the same library.
Rich Nagel published this list of supported messages: http://www.mudlord.info/bassmididrv/BASSMIDI_Driver_MIDI_Implementation_Chart.htm. It will apply to VirtualMIDISynth too.

As for the "soundfont chain", think it like an ordered list where the last loaded SF wins.
For normal use you could load a single (your preferred) soundfont and use it for all instruments.

But some users don't like some instruments contained in a SF2, so they can change it with (better) others.
Suppose you configure a full GeneralMIDI soundfont at first position (A.SF2), then a small SF containing only your preferred Acoustic Grand Piano samples at second (B.SF2).

Now VirtualMIDISynth will use A.SF2 for all instruments except for Acoustic Grand Piano which will be rendered from B.SF.
This chain could contain up to 10 items.