I always used the yellow copy of the invoice for my chemical log. Check the box to indicate the chemical used, (target pest was pre-printed: weeds, crab, insects). Gals used. Also time and date, were recorded, and wind conditions were checked as low, med or high. My customer cards had pre-printed amounts for the pounds of fert needed or gals of solution needed.
Yellow copies were used to post charges for statements and copies filled. Chemical data was entered into a database in winter.