Yeah, that's confusing. UHP is ultra high pressure. They are mercury vapor. From what I am reading, there are also mercury vapor bulbs labelled SHP, for super high pressure. However, this particular product does look to be HPS.
Here is an example of a bulb that would fit in it and is made for growing plants:
http://www.luxlight.de/en/sylvania-s...0w-grolux.html