In Flow Science, they provide an example of boundary conditions and how to specify them for CFD simulations. The following is stated:
Pressure Boundary Example
For example, consider the problem of flow in a section of pipe. On the one hand, if the upstream end of the computational region coincides with the physical entrance to the pipe then a stagnation condition should be used to represent the external ambient conditions as a large reservoir of stationary fluid. On the other hand, if the upstream boundary of the computing region is inside the pipe, and many diameters away from the entrance, then the static pressure condition would be a more reasonable approximation to flow conditions at that location.
which I don't really understand, because inside the pipe, the fluid is already in motion, how come we shall use the static pressure?