They're used for both. Their purpose is to seal off heat, because you get a lot of heat once you start using 400w lamps and higher.
An HPS is the favorite choice for flowering because it has lots of light from the red spectrum, autumn light, but can be used for veg too, though not the favorite light for that.
A good idea would be to mix in a few cool fluors though to make it more suitable for veg.

An MH is the favorite for veg, because it has light from the blue spectrum, which simulates day light/spring light.
I don't think the MH is as all round as the HPS though, I doubt that people flower under an MH.

Fluors are good for veg too, though perhaps not the pro choice of method.

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