The usual kinematic boundary condition is that the flow normal to the boundary element is zero. There have been designs for aircraft wings in which air is pulled into the wing in order to help avoid stalling in order to obtain greater lift. In this case the normal flow (either in terms of velocity or volume flux in incompressible flow or mass flux) can be specified where there is flow through the boundary. Another example is flow in a river where there is infiltration into the soil so that there is actually flow normal to the bottom boundary. Similarly, often in coastal areas at the beach, there is freshwater coming out from the sand having flowed through the land mass.

Thus, flux of volume or mass can be specified at the natural boundary.

The prescription of the normal velocity is enough for a potential flow but is not sufficient for a flow with vorticity

For a set normal velocity (or set dynamic pressure) inflow you must also specify the tangential vorticity or tangential velocity (to complete the advective term in Navier-Stokes).

For a set normal velocity outflow (or set pressure) it is possible (numerically stable - though marginally physical) to upwind the tangential velocity or tangential vorticity and get values from the interior. Mathematically - this is setting those variables. Practically, the user does not need to do anything (so they see the BC as 'undefined' - at least by them. This is effectively very close to using zero normal derivative for the tangential velocity or vorticity.