Piping thermal loss coefficient. CSP Trough Physical, Commercial

Dear Paul,

I wanted to ask you about this parameter in the help of the program. In this parameter, it says "The coefficient specifies the number of thermal watts lost from the system per pipe surface area, square meter of aperture area and temperature difference between the fluid in the piping and the ambient air".

Is it "per surface area" or "square meter of aperture area"?
If it is per surface area, is the exterior area of the isolation or the interior area?


Thank you very much.

With best regards