Trim controls are used to adjust and level the output of whatever channel/s your trimming. Since receivers generally have either individual trim controls for each channel or trim control for left/ight, surrounds, and rears you would use them to balance these levels. The volume control effects all channels levels proportionately.
Turning up the trim of say the mains, at a set volume level would increase the output you hear, but the actual volume level in terms of the postion of the control, remains the same.
Bottom line is, think of trim controls as gain controls used for minor adjustments & channel balancing, and the volume knob as your primary system level control.
