The QA351 is a low-cost voltmeter providing microvolt resolution and quick programmability through its REST interface. The QA351 should not be expected to replace a $2500 6.5 digit DVM. Instead, the QA351 makes sense in applications where you are primarily concerned with precise, relative measurements over days or months in situations where humidity and temperature are reasonably controlled (for example, an office setting).
The QA351 provides two channels of measurements: A high-resolution main channel (uV resolution) and a lower-resolution aux channel (mV resolution). Both of these channels are differential. The outer shell of the BNC connector is the low side of the measurement, and in the inner conductor is the high side.
The QA351 is NOT designed to measure high-energy circuits. High-energy circuits are circuits that are connected to the electrical grid. The QA351 is a CAT I device, which is the "weakest" designation for measurement devices. This means you must take external measures to ensure transient events (such as a lightening strike to the utility lines outside of your office) are clamped upstream of the DUT and are NOT propagated through the device you are measuring to the QA351.
The front panel of the QA351 features a high-quality 2.5V reference (± 500uV, 3 PPM/C tempco) for calibrating the QA351 and testing other DVMs in your lab.