P = Processor and S = Socket (they're pretty interchangeable). U = rack Unit https://en.wikipedia.org/wiki/Rack_unit

n-P / n-S / n-way = how many sockets/processors a system has. A 1S system has one socket / processor, a 2S system two, a 4S four and so on.

x U (or x HE, if you're talking with a German manufacturer, they like to make that mistake ... ;) are rack-units, i.e. how large the case is.

The title should be re-written to say "single and dual socket" not "1P and 2P".