Intel started this low-voltage brand a while ago, but with the third generation of Core processors, better known as Ivy Bridge 22nm they decided to do some renaming. The ultra-low mobile brand is now gone in favor of Y Processor line but in the retail market they end up with U as the suffix, as consumers have to have some way of telling that they are using a low power part.
Mobile processors, including the Core i7-3517U, have a TDP of 17W, but apparently Ivy Bridge is capable of hitting 13W in Y-series parts. The Y-series features a BGA package and the SDP (Scenario Design Power, not social democratic party Ed.) stands at 7W.
Intel explains Scenario Design Power is an additional thermal reference point that is meant to represent mainstream workloads for a tablet-first experience. So this means that Y series will find its way to tablets and convertibles, rather than real notebooks or Ultrabooks.
Haswell can sink the TDP further, as its Y processor line, also in BGA can bring a TDP of 11.5W, with SDP down to 7.5W, just half a watt more than with Ivy Y, series but overall better.
The U processor line with Haswell will land at 25W TDP for some parts and 15W for the Ultra-low mobile parts that we all like in Ultrabooks. Apparently SDP is a reference used for tablet, and not for any other mobile processor.