DC offset is adjusted for minimum DC on the output. The level trim adjustments are just that. Apply a similar signal to each input and adjust so that the outputs are equal.
Adjusting Infinity IRS servo controller
I got a request from @bobterry99 about adjusting Infinity servo controllers. Inside the controller are 4 pots:
Left side level (amplitude)
Left side DC offset
Right side level (amplitude)
Right side DC offset
See the attached photo. I used a 2 ch oscilloscope to monitor the input and output while adjusting. With 4 ch, you could monitor both channels...