iOS/GPS altitude
Altitude estimated by the device's location service. It works reasonably well in most situations but can't track small changes and can be unreliable indoor.

Barometric altitude (iPhone 6 minimum)
Altitude based on the device's barometer. This is how most hardware altimeters work. It can track very small relative variations but must be calibrated with weather data or GPS. Calibration with weather data grabs current sea-level pressure and temperature at your location from a nearby weather station. GPS calibration takes the current iOS/GPS altitude as a starting point then uses the barometer to track changes.

Elevation database
This method displays earth surface elevation at your location by polling an elevation database. It works even with very imprecise location data but requires network access. Surface elevation usually includes buildings and trees.