As shown if you extend the face of the one cylinder bank out (perhaps with a flat plate or something) it's giving you the measurement to the corner of the other bank. This will be important when you fit the intake manifold on the top of the block. If it's not correct the monifold won't fit properly. I guess if you do everything else correct this measurement will be automatic. But it can / should be used as a double check and probably should be laid out on the end of the block as a guide to be sure you hit it right on doing the other machining. Not sure about his orientation suggestion. Whatever way you choose to get everything at the correct angles is the right way.