So im no expert ive been learning mixing as i go...why the gain knob above the fader on each channel ? How and when would it be used ?

It's used for gain staging purposes. If you have a recording that was recorded too hot and you need to hit your first plugin at a lower level (let's say for instance you're using an analog modeled plugin which requires an input between a certain range), then you can use the trim knob to bring the level of the track down before it reaches any plugins. Likewise, if the recording is too quiet then you can bring it up. 

You may find this hepful

 




 

This is one of the most important knobs, you're ever going to play with lol true story. It's as Sharke suggests, it's used for fixing your gain. But you can use it more broadly, however, for example before you begin a mix you could make sure every track is sitting at roughly -18db. This is a good way to learn mixing if you're a newb. Another use, altgough I use Wave's gain knob 😎 in their NLS plug, but I also use the gain knob 🤣 to finalize the level for the master buss. It's almost impossible to not go over the 0db mark otherwise. Mixing, and creating headroom is also key to getting bigger mixes too. Ignore this advice if you mix and master and send signals outside the box when doing so. You can also use the gain knob 😎 to set levels to Aux and Buss tracks.

Be careful with the midi gain knob though as it's name is misleading. It's Midi Velocity, not volume and s will change the tone of the softsynth. Which can be useful, but if you turn down then low velcity notes flatten to 0 and turning up. high velocity notes flatten out at 127.