It is written in this article: http://www.maths.ed.ac.uk/~yktsang/4520/basic_fluid.pdf "In the continuum model of fluids, physical quantities are considered to be varying continuously in space, for example, we may speak of a velocity field ~u(~x, t) or a temperature field T(~x, t). The “local” values of such quantities at a single point P in space should be understood as average values over a small region of size Lp about P."
Then in the example he took a small volume Dx and Dy: " We now consider the various forces acting on a fluid element of sides ∆x, ∆y and ∆z in our channel flow problem"
My question is, this fluid element is it equal to Lp? If no, does the pressure in his study is an average value over a small region of size Lp ?