This has been discussed so many times on the forum, and of course it's clearly explained on the manual.
- At Page 11, it says the passenger number is by default estimated by dividing the total station load by 220 lbs.
- At Page 29, it explains that, by disabling the "Estimate Passenger Number", you can disable this default calculation and enter the number directly.
- Some airplanes, like the FS Labs Airbus, can control the number of passengers in GSX, to synchronize their own progressive loading process with the GSX passengers boarding, which will affect weight and balance in realtime, while they board.