"it_icon_adjust" is how you want the game (BoA) to interpret the graphic that you choose. If you played the sample scenarios, you saw some color variants of some different graphics, or they were darker, lighter, tinted, inverted, color-swapped, etc (or a combination of a couple). That's what that it is. It just doesn't show up in the editor unless you also use "it_ed_icon_adjust". If it is not called or left at -1 I believe that no adjustment is made. (Similar calls are "cr_icon_adjust", "fl_icon_adjust", and "te_icon_adjust".)
(This information is dependant on whether I have read the instructions correctly.)