Hey Ian,
Since the option between unidirectional and bidirectional devices is so fundamental I can only point to specific sections of documents which explain it. Essentially, unidirectional means the current is only flowing in one direction while bidirectional means current can flow in both directions. It is up to customer on whether they want to measure one direction of current or both, but if they need to measure both directions (positive and negative current), then they will need a bidirectional current sense amplifier where the reference pin is broken out. A reference voltage (Vref) is needed to offset the output voltage typically half-way in between GND and VS. Unidirectional parts short this REF pin to GND internally.
This means that VOUT = I*Vshunt*Gain + Vref for bidirectional devices.
This technique is needed for any amplifier operating on a single supply voltage.
Here are places where it is discussed:
Sincerely,
Peter
