Altitude indicator options
In addition to showing GPS altitude above sea level (#1), from version 1.9.3 you can show altitude (#2) and height (#3) that are based on a barometric pressure. Given your device has a barometric sensor (all recent devices do).
To show altitude or height based on barometric pressure you need to establish a reference between pressure and altitude.
Altitude indicator menu offers an option to use current GPS altitude and air pressure as a reference (#5) or you can use “Set complete altitude/pressure reference” option (#4) to setup arbitrary correspondence between altitude and pressure:
Calibration options include setting altitude and corresponding to it Surface (QFE) or Mean sea-level pressure (QNH).
I hope I studied enough to assign QFE and QNH abbreviations correctly here :). You can always write me to email@example.com and I’ll be happy to fix any injustice to aviation terms and functions.
If you pick a barometric source of altitude or height, altimeter indicator will give you a hint, saying BARO ALT or BARO HGTH: