The pressure sensor is used only for SSR traffic (Mode-A/C/S) and ADS-B, unless you have a Mode-S transponder, in which case it will use your Mode-S reported altitude instead. The pressure is used to compare the own altitude with the reported altitudes of other transponder and ADS-B aircraft. The pressure is also used to log the altitude in the IGC file (used by gliders). The pressure sensor can be calibrated using a calibration center, which will results in a calibration report. No calibration values are programmed in the device.
For collision avoidance with other FLARM equipped aircraft, FLARM uses only the more accurate GPS altitude difference.