Where U is the tip speed of the turbine wheel and the C is the velocity of the exhaust going into the turbine. This is important because it helps you select a A/R ratio for the turbine and impacts turbine blade design. You can gear a turbine wheel for different U/C ratios (basically to make the best use of low gas speeds or high gas speeds) and help set a turbo up for higher or lower speed operation. So in the PDF link where they are talking about heavy EGR use siphoning exhaust gas out of the manifold and lowering the volume and velocity of the gas going into the turbine, there arises a need for a turbine designed with maximum efficiency at low gas speeds.
