It is midi volume.
Go to your midi track inserts and insert the midi monitor. Play your midi part back & move the fader - you’ll see the volume CC’s displayed in the midi monitor. However, if you don’t move the fader, what you are seeing in terms of the “pulses” is a midi event - usually the note velocity if you’re just watching the playback. Try playing a few notes, go into key edit & draw a velocity ramp. Play back the midi part and watch the values in the midi fader change to their max value.
Personally I never or rarely display midi tracks in my mixer. I use the filter to turn off the midi channel in the mixer. It saves a lot of mixer clutter & gives you more “real estate”.
If you do keep the midi tracks displayed, I’d be tempted to leave the faders at zero. After all, you’re probably and most likely going to mix through the instrument tracks anyway
Hope that helps
Neil