These two modules are both bought with the same in-app purchase. They also share the same calibration. Once you have calibrated either one, you can use the other.
The Impedance Meter is used to spot check impedance at any frequency, while Impedance Sweep will draw a plot of impedance by frequency.
The Impedance tests use the output of iAudioInterface2, but putting the device in a special loopback mode. Other interfaces cannot be used for impedance testing.
These modules are designed primarily to measure low-impedance voice coils, and are most accurate in the 2-200 Ohm range. Above 1000 Ohms the accuracy drops off quickly.