The iOS device's built-in microphone supports units of FS (full-scale, or maximum input before clipping) and Pa (pascals). It does not support any other engineering units, like the headset or dock inputs do.
The default pressure sensitivity of the built-in mic is defined for each iOS device model which should get most iOS devices within 3 dB of the actual sound level at a frequency of 1 kHz.
The pressure sensitivity of the built-in microphone is synchronized with the device sensitivity (the two are always the same, so calibrating one calibrates the other).
It is possible to adjust the input gain of the built-in microphone for some iOS devices. In this case, SoundMeter offers Low, Mid, and High gain settings with nominal sensitivity values for each setting. This means that the gain can be changed and the level measurements retain their accuracy. User calibration can be performed for each gain setting independent of the others.
The Gain setting can be used to adjust the measurement range. To measure higher signal levels, choose a lower gain setting.