The static pressure (termed the Supersonic/Initial Gauge Pressure) must be specified if the inlet flow is supersonic or if you plan to initialize the solution based on the pressure inlet boundary conditions. Solution initialization is discussed in Section 22.13.
The Supersonic/Initial Gauge Pressure is ignored by FLUENT whenever the flow is subsonic. If you choose to initialize the flow based on the mass flow inlet conditions, the Supersonic/Initial Gauge Pressure will be used in conjunction with the specified stagnation quantities to compute initial values according to isentropic relations.
Remember that the static pressure value you enter is relative to the operating pressure set in the Operating Conditions panel. Note the comments in Section 6.3.1 regarding hydrostatic pressure.