IOScope can acquire signals from the current audio input path (the iPhone OS automatically selects the audio input path according to which external audio devices, if any, are connected to your device).
Different engineering units, including Volts, Amps, pascals, g's, and ips (inches per second) can be assigned to input channels of the headset input (not available on 1st generation iPod touch devices) or dock connector audio input. Sensitivity calibration can also be performed for any audio input device or input channel.
One reason for including separate units and sensitivities, for the input device and the input channels, is to enable a voltage calibration at the device level, which then allows you to enter a standard sensitivity for a transducer connected to a particular input channel (e.g. 50 mV/Pa for a microphone or 100 mV/g for an accelerometer).
Assigning units to, and calibrating, the two input channels is useful for measuring impedance (in ohms, or V/A) or device sensitivity (such as V/Pa).