I used to have a link for digital ballast light usage specs but it's long gone.
Let me try to explain how a digital ballast works. It can use both MH and HPS bulbs. See, wasn't that easy? hehe
Seriously tho, the companies that make bulbs still have not been able to make a true 600 watt Metal Halide bulb. You can use a 400 hps or mh in a digital ballast. You can use a 1000 watt hps or mh in a digital ballast. But the 600 watt is only made as an HPS bulb. They add a special part inside the 600 watt hps bulb to create the mh apsect. This is why they call the 600 watt mh a "conversion" bulb. It is actually an hps bulb converted to mh thru the ballast.
I actually like to use the 600 conversion mh bulbs instead of the regular 600 hps. I've done head to head and the 600 mh is better hands down. The downside to these bulbs is that they burn out twice as fast as 600 hps and don't give anywhere near the lumen output of the 600 hps either. But for some strange reason they are better than the 600 hps and have almost 15k to 20k more lumens than the 400 mh.