why does your textbook use U for velocity instead of V
there are many symbol conventions and it is hard to argue that one is right or better than others. although many intro fluids textbooks use \vec{v} and V for velocity or speed, advanced fluids textbooks are more likely to use \vec{u} and U and the books I learned from all used some form of u.
Using \vec{u} is a little bit more natural IMO with the use of u v and w for velocity components, but maybe that’s just me thinking that it is natural because it is how I learned it.
Using some form of u avoids confusion with volume V I suppose, and using some form of v avoids confusion with internal energy U.
I think it is basically arbitrary.