Accurate flow directions are more important when the flow is into the model, while they are of less importance at the outflow. This is because any errors at the inflow boundaries are transported into the model and may, therefore, cause instabilities.
Introducing open boundary conditions into a finite difference model is a very complex task as a number of different implementation solutions can be used. The input description as given above focuses on the description as seen from the user's practical point of view (“these data are available; how do I specify that to the model?”), while the description as seen from the numerical point of view is given in the Scientific Documentation.