Pressure inlet definitions
 

Hi. I am having difficulties understanding the pressure inlet box. See below a copy of the pressure definitions:

"Gauge Total Pressure sets the gauge total (or stagnation) pressure of the inflow stream. "

and

"Supersonic/Initial Gauge Pressure sets the static pressure on the boundary when the flow becomes (locally) supersonic. It is also used to compute initial values for pressure, temperature, and velocity if the pressure inlet boundary condition is selected for computing initial values."

My problem is the supersonic gauge pressure. The boundary described, is that the inlet boundary or the boundary where the flow becomes supersonic? If it is the latter, I fail to see from where I should know this information. It is this answer I am seeking to calculate using Fluent, not the other way around. And from where should I know the gauge pressure (gauge or stagnation, btw.) when I don't know the velocity.

Help is appreciated.