Interfacing

[usul]

Hello

First of all, I'd like to thank you for this excellent sofware and for the integration of navigraph charts.
I'm writing to you because I'm looking for a solution to interface the software with my cockpit.
I would like to be able to operate the buttons on the GTNxi 750 using the buttons on my cockpit: 
Home 
direct to 
dual rotary knob
volume/squelch knob

In your documentation you give MSFS events (K events) 
GPS_PAGE_KNOB_DEC                         Bottom Right Inner Knob Decrease and
GPS_PAGE_KNOB_INC                          Increase where:
                                                          0 = GTN750Xi Unit 1
                                                          16 = GTN750Xi Unit 2
                                                          8 = GTN650Xi Unit 1
                                                          24 = GTN650Xi Unit 2

I don't understand these lines:
0 = GTN750Xi Unit 1
16 = GTN750Xi Unit 2
8 = GTN650Xi Unit 1
24 = GTN650Xi Unit 2

What do 0, 16, 8 and 24 stand for? How do I use them?
I use spad.next and I know how to modify LVARs. But I don't understand these numbers.
An example would be great
Thank you for your answer.