AB has the concept of a Device Level Ring (DLR), where you can connect Ethernet I/P devices in a ring topology. In order for this to work, each device needs Ethernet ports that are designed to be connected in this way. Not all devices have 2 ports, so they make an ETAP for this case. The ETAP has the 2 ports, which allow it to be connected in the ring, plus the 3rd port for the device.