What my previous comment states is that the operating pressure in not used as a boundary condition to solve for the flow field. It definitely has a meaning as well as significance in the problem. The operating pressure that you specify serves as a reference value which you use to compute the gauge pressure to be entered at the pressure-outlet boundary(from the absolute pressure data you have).nSuppose, the problem that you are working on is under atmospheric conditions. Hence, the operating pressure for your problem would be 1atm(which is the default value in Fluent). Now if you have the velocity data at the inlet, you don't need to specify the pressure at the inlet. However, at the outlet, since you have the data for absolute pressure(1 atm), you select pressure outlet as the boundary condition and specify a gauge pressure of 0 atm(absolute pressure - operating pressure). nIn case of incompressible flows, you can specify different values to the operating and gauge pressure(keeping the absolute pressure at the boundary the same) and still get the same results. For example, in the above case, you can set the operating pressure to 0 atm and the gauge pressure as 1 atm(hence, absolute pressure=1 atm), and the results wouldn't be affected. However, in case of incompressible flows, this can give incorrect results as the pressure governs the density of the fluid.nHope this clears your doubt. For more information, you can refer Fluent's User guide.n