Both high speed and transistor output optocouplers can be used in PoE applications. The power sourcing unit might have an I²C bus or some other interface between the controller chip and the power control chipset which requires isolation. The data rates used require high-speed optocouplers. Isolating the address line from the controller chip doesn’t require high speed, and standard transistor output optocouplers can be used. In the powered device, such as an IP phone, there can be a DC/DC converter in the feedback loop and isolation can be accomplished with transistor output optocouplers.