It is often confusing to know if you should use this
I have included this because most printers can not print right to the edge of a sheet of paper. Typically they can not print within 0.25 inches of the edge.
If you specify the trim area, the target generator will shrink the printed area by that much, and will also reduce the target scale by a corresponding amount. Examples:
For some targets, such as a british 1887 figure target, features of the target go right to the edge of it. On 8.5 x 11 inch paper, printed, the very points of the figures will not be printed by a printer that can not print to the edge of the paper. since a large portion of the "end" of the figure is cut off, the target "looks funny" and with trim, the target looks better, and you slightly change the distance you shoot at. For a small target that is reduced a lot from the original, this may be what you end up wanting.
Most of the time you probably will NOT want to trim large sized targets.
Considering the same british 1887 figure target, if we were printing it full size (or close to it) we probably care much more about the inner scoring rings being the correct size, than if the outer edge has printing on it. If we are scoring a hit for outer anywhere on the paper, the *paper* is still the correct size for our scale, and corresponds to the printed target inner ring sizes even if there is a small outer band the printer could not print. The target will also not look as "funny" because in the example of the british 1887 figures, proportionally, taking 1/4 inch off the top of a 48 inch tall figure is much less noticable than taking 1/4 inch off the top of an 8.5 inch figure.
Happy shooting!