The term "Trim" can be applied in several related ways.

Here is an excerpt from a Lincoln PowerWave operators manual

VOLTS / TRIM:
In constant voltage modes (synergic CV, standard
CV) the control adjusts the welding voltage.

In pulse synergic welding modes (pulse GMAW only)
the user can change the Trim setting to adjust the arc
length. It is adjustable from 0.500 to 1.500. A Trim setting
of 1.000 is a good starting point for most conditions.

Power Wave 455/STT Only: In STT modes, the user
can adjust the Trim setting to change the overall heat
input to the weld..
http://content.lincolnelectric.com//pdfs/products/navigator/im/IM716.pdf