FAQ: Logic and Voltage Translation > Logic Technology >> Current FAQ
The "Ioff" specification in the electrical characteristics table is only in the datasheet of devices that have the special feature of "partial power down" - also known as "back-drive protection."
Partial power down protection is the lowest level of live-insertion isolation, as defined by the application report "Logic in Live-Insertion Applications."
Partial power down essentially states that, if the supply of a device is held at 0V (any supply for multi-supply devices), then the input and output pins of said device will enter a high impedance state. The maximum leakage current in this high-impedance state is defined by the Ioff specification in the datasheet.