why this type of boundary condition works?

manish

I have read ijn cfx manual that when we apply mass flow or velocity at inlet and pressure at outlet then it will more robust.but when we apply pressure at inlet and velocity at outlet it will diverge the solution.The later is applicable only to compressible flow.Can anybody tell me why this is happening? the outlet mass flow is applicable to fully developed flow.so what is fully developed flow? and why it is robust for outer mass flow? Thanx