540 rpm is standard speed for pto implements. If the implement is very light to run, like a fertilizer spreader, you can use 540e, which is actually 750rpm at the same engine RPM that would give 540. That means you can use much less throttle on the tractor, saving fuel and noise, and still get 540 rpm on the pto shaft. So for instance in 540 you would run the tractor on 2000 rpm, while in 540e you can run it in 1500 rpm, and still get the right rpm on the pto shaft.