It's actually a bit unclear, as page 23 of the manual states if "estimate passengers numbers" is disabled:"
That sentence started with:
This option is only relevant on FSX/P3D. In MSFS, the default is to always Estimate the passengers number, because we added Simbrief integration, for a more realistic and precise set of the passenger number.
The first paragraph:
In MSFS, the default is to always Estimate the passengers number, because we added Simbrief integration, for a more realistic and precise set of the passenger number.This implies that because of added Simbrief integration, "Estimate Passenger Number" is set to ON
by default as with this option ON, GSX will read the Simbrief passenger number - unlike in P3D where it was roughly estimated based on payload weight.
The unclear bit is what GSX would do if I manually set it to OFF.
The second paragraph:
If this option is disabled, GSX will always ask confirmation from the user about the number of passengers onboard.Is there any user confirmation prompt about the number of passengers onboard when "Estimate Passenger Number" is set to OFF? Unlike in P3D when you can manually enter passenger numbers, I certainly don't see it in MSFS.
I'm guessing that this part of the manual wasn't written correctly, I suspect if "Estimate Passenger Number" is set to ON, passenger number is estimated via payload weight, like in P3D. if "Estimate Passenger Number" is set to OFF, GSX reads data from Simbrief. If Simbrief data is unavailable then it goes back to estimating passenger number based on payload weight. It looks like the manual entry of passenger numbers, like it was in P3D, is no longer possible with GSX for MSFS.
Or alternatively the "Estimate Passenger Number" option does nothing in GSX for MSFS, and that GSX will always read Simbrief data first, if unavailable then it estimates passenger numbers based on payload weight, with no possibility of manual entry. This happens whether the option is ON or OFF.