"LP system" is really a poor descriptive term. What people usually are implying by such is a combination of a higher flow valve and bolt, often a volumizer too, and low pressure springs to produce the right velocity. The good that such does is measurable but unimportant on most guns, and isn't really all that cost effective for the benefits received. Cockers are really the only guns that such is practical with, as a piranha or similar can be enhanced but at a cost of half to nearly 100% of the price of the gun, which upon resale would not likely be regained. In short, for anything but a Cocker it's a poor investment.
HPA or high pressure air is another term for a nitrogen tank, as it can be filled with compressed air from a scuba tank or directly from a compressor, which is functionally the same as filling it off a tank of pure nitrogen.