This is to make it clear that the value supplied is the scalar mean velocity normal to the patch, and to distinguish it from other instances of the keyword "UMean" which take a vector quantity.
Reference: Duisburg Test Case (DTC) el Moctar, O., Shigunov, V., Zorn, T., Duisburg Test Case: Post-Panamax Container Ship for Benchmarking, Journal of Ship Technology Research, Vol.59, No.3, pp. 50-65, 2012