As the altimeter gauge works relatively to a pressure change, it needs a calibration level on the ground. Also, to synchronize settings in the air, all planes must use a standard calibration level high up.
You should set your altimeter gauge for proper pressure level. Before you start your engines, press B and it will calibrate your altimeter to the departure airport. If you pass transition level, or FL180 max, press B again, so it will calibrated to standard 29.92 HgIn. If you go below FL180, or near the destination airport, press B again, and it will calibrate your altimeter to the departure airport height.
If you fly offline, you can listen to ATC, and they will always tell other pilots too the correct pressure, which you can set manually too on your altimeter gauge.
If you fly offline, with a static weather, you do not need to calibrate altimeter. If you fly online expect rapid pressure changes, as in real
