The Velocity Input/Output panel is there to help you control the ratio between the loudness of your playing and the sample playback volume. This tool is useful in a multitude of cases, but here are some common uses:
Make your sampler sound at a single volume regardless of input velocity
In order to make a sampler ignore the dynamic range of your playing and just sound at one volume, you can drag the bottom left tab of the velocity panel straight up to match the level of the top right tab. Now even a very low velocity pad activation will cause the sampler to sound at full volume. If that is too loud for you, you can just lower the volume knob.
Stop Sympathetic Vibrations from Activating Choke Groups
Activating long, often tonal, samples with a single hit and having samplers choke one another is one of the things that makes electronic drumming interesting and fun. Sensory Percussion allows you to layer harmonic and/or melodic sounds over percussive sounds, which is one way to create complex soundscapes, but inevitably leads to a dilemma: Most likely you will want your long tonal sounds to behave differently than your short percussive sounds, i.e., you'll want your tonal sounds to choke each another, but not be choked by the percussive sounds underneath. And you'll want your percussive sounds to be blended and perhaps even respond to sympathetic vibrations from a hit on another drum. That is one of the most unique things about Sensory Percussion - it responds like an actual drumset if you enable it to: each pad will activate at a low volume if it is stimulated by the vibration of another drum, just like an entire acoustic drumset vibrates when just one drum is struck.
This sympathetic vibration is great for percussive sounds, but if you have tonal sounds in choke groups, the cross-talk from one drum could choke your tonal sounds by activating a sampler at an inaudible volume. Before heading straight to the threshholding panel, which will stifle all cross-talk and sacrifice the sympathetic vibrations of the percussive sounds. You can handle the cross-talk on the sampler-level which will allow the percussive sounds to respond naturally to cross-talk, but not choke the tonal sounds layered overtop.
Adding a sampler to a choke group will automatically remove it from blends and move the left velocity IO tab slightly to the right, but if your drums are very close together or very resonant you may find that cross-talk from one drum still activates a choke group on another drum. In this case you can move the left velocity IO tab even further to the right.
This will also allow you to explore activating the choked tonal sounds more delibrately at higher velocities, leaving the percussive sounds to play solo when you play at lower velocities, which adds another layer to your playing.
Make the sampler ignore high or low velocity hits
It may be useful for you to have a sampler that ignores either high or low velocity hits. The use case for this would be that you have one sampler that sounds on every hit, but another that you want to be silent during loud playing. In order to accomplish this you would drag the velocity panel's top right tab to the left.
If you want the opposite to happen (sampler doesn't react to quiet playing), then you can pull the left tab to the right.