Can VIN have voltage when VOUT is applied with 5V and VIN is not applied?Can it be used as a bidirectional converter?
Can only be used as a step-down converter. Not a bidirectional converter.
but what does it mean to sink current?
The sink current is well explained in the datasheet in the applications section.
Connect the 20V battery on the left and the 5V battery on the right. Can the battery on the right charge the left side?