On the 425, at least on my 2001 model year, the AUX is the 3rd function on the FEL bank of valves that is downstream from the power beyond port on the steering valve. It operates the QA and any attachments with cylinders like mini-ho and power angle snow plow. I think that's only rated at 4GPM and the pump section that feeds all that comes on as soon as you crank the engine, as it's physically connected to the crank shaft and there is no clutch.
The MAIN PTO is always on as well. That is the one that's rated at 8GPM. It's physically connected to the crank shaft as well. That's the one with the electric diverter that switches between return directly to tank OR go out to the FEL arm in/out connectors for the mowers and such.
Not sure how the larger units are plumbed, but I'd suspect similar, and you'd have a heck of a time trying to hold a hose in a bucket while cranking the key, throttle, diverter switch, AUX lever, etc... big mess coming.