I agree with what David wrote above. Just to clarify a few things:
The Trim Knob is not a volume control. It's a variable pad meant to tame low impedance source input levels. As it turns counter-clockwise, the Input's sensitivity decreases. If it were a switch instead of a knob, it would be labeled "line" on the left and "mic" on the right, and have two midway positions labeled "mic -40dB" and "mic -20dB".
The Trim Knob impacts the signal level at the Input, the track record level, and the track meter. But the source connected to the Input is the primary control over of the signal level being recorded. The trim knob is there to reduce the level of a hot low impedance signal.
The trim knob generally should be set full right for low impedance (lowZ) sources (mics), and full left for high impedance (hiZ) sources (outboard effects returns, pre-amps, etc.).
[Guitar pickups are HiZ - typically 40kHz. The "Guitar" setting on Input "H" changes the impedance on Input "H" to an extremely high 1M Ohms. Since it is high impedance, the initial Trim Knob setting should be full left. Adjust the guitar's volume to control input level for recording. Some single pickup guitars have a weak signal. If that's the case, turning the Trim Knob clockwise to increase the Input's sensitivity can bring the incoming signal up to an acceptable recording level. If this causes no discernible impact on the recorded sound (any kind of change in the guitar's sound, loss of higher frequencies, distortion, etc) then this is the one exception where the Trim Knob could be used to increase the weak line level signal for recording purposes.]
If a lowZ source is overloading the Input, the first option should be, for a mic, to move the mic further away from the source. If that doesn't solve the problem, ~then~ use the Trim Knob to manage the Input signal.
If a hiZ source is overloading the Input, reduce the output level of the hiZ source.
The proper way to raise a weak incoming signal is, if a mic, bring it closer to the source; and if a line level input, raise the output level of the hiZ source.
The track fader controls the volume of the signal going to the bounce bus and to the output busses. It has no impact on the Input channel signal, track meter or the track record level.