Barometric Alimeter use the integrated barometer device of the iPhone 6 and iPhone 7 to display the current altitude in meter or feet.
When the barometer is not avalaible for exemple if you have a iPhone 5 or iPhone 4, the application use the GPS to display the altitude.
In the settings page you can choose the calibration method (GPS, sea level or manual) and you can change between the two units systems (metric and imperial).