PUR = photosynthetically usable radiation, a concept I haven't yet seen a good reason to dig into. Plants use light from the whole spectrum between about 400 and 700 nanometers wave length. Unlike what many people believe, they also use green light, just not as efficiently as they use blue and red light.
If it was hard to get enough light to grow our plants, we would want to get every advantage we could, and look for bulbs that wouldn't waste any light in the green wave lengths, which, if you think about it, would leave us with very drab looking plants. But, the bigger problem we now have is avoiding too much light, not getting enough light. So, why bother with PUR?