Wow, I never even knew that was a thing. That is so much more detailed than I would ever need and/or use. It may have a place in many of our members projects… but that depth of control and understanding wouldn’t be something I would ever need to personally use.
Some programs/instruments including Groove Agent can use multiple samples to more accurately model a light vs a heavy keystroke or drum hit. As Easto noted, the velocity range in the MIDI standard is 0-127.
The specific example below in GS SE6 (“Hi Smack L”) has four samples for different velocities. The sample highlighted (“02_PC11_HiBongoSmack_V1L..”) will be activated when the keystroke/pad/whatever is being hit with a 0 to 31 velocity (numbers on the far right). If you click on the other samples the corresponding velocity range will be displayed.
Some kit pieces only have one velocity sample, others have more. I don’t know the maximum number of samples but I seem to recall loading 12 for one drum when I was experimenting a while ago.
This information is only available in the “Beat Agent” kits, which are designed to be customized, edited, added to, etc. The “Acoustic Agent” and “Percussion” kits do not have the same screens for editing - they seem to be based on a more closed “use the kit as-is” idea.