The AB line you create is the basis for all of your field operations. The tractor (if you are using auto steer) attempts to drive perfectly on that line. Depending on what your implement or swath width is set at, the program then draws perfectly parallel AB lines to the one you created at that width distance from it (or the adjacent line) so as you reach then end of the field and turn around or use auto u-turn it will drive the next AB line back across the field. Does that answer your question?