here in BC, the requirements specify a "truck tractor" or "a vehicle with an overall width of 2.05m [80"] or more, or a trailer or semitrailer having a gross weight in excess of 1,400 kg" must display clearance lamps, one at each corner [amber front, red rear], or as near as possible, and "commercial vehicles" may display 3 identification lamps, near the top, front and back, no position spec stated, but they're always as cren sez, 6-12" apart in the middle
folks get confused about those cab lights, the outside ones are the "clearance lights" req by law, the middle 3 are "marker [or ID] lights", any on the side are "side-marker" lights - they serve to identify the size and position of larger trucks at night or in poor visibility and really shouldn't be positioned any way other than as specified
Bill
