The specification for IO-Link distinguishes between two port types: Port Class A (Type A) and Port Class B (Type B).
In Type A the functions of pins 2 and 5 are not specified. These functions are defined by the device manufacturer. Pin 2 is typically assigned to an additional digital channel.
Type B offers an additional supply voltage and is suitable for connecting devices that require additional or separate aux-power. Here pins 2 and 5 provide an additional (galvanically isolated) supply voltage. A 5-conductor standard cable is required for using this additional supply voltage.