Bidirectional operation allows the INA28x-Q1 to measure currents through a resistive shunt in two directions. In this case, the output can be set anywhere within the limits of what the reference inputs allow (that is, from 0 V to 9 V, but never to exceed the supply voltage). Typically, it is set at half-scale for equal range in both directions. In some cases, however, it is set at a voltage other than half-scale when the bidirectional current is nonsymmetrical.
The quiescent output voltage is set by applying voltage(s) to the reference inputs. REF1 and REF2 are connected to internal resistors that connect to an internal offset node. There is no operational difference between the pins.