This read-write object specifies whether the optical plug on this port is acting as the head-end or the tail-end of an optical link, defining its role in a point-to-point optical connection. An administrator configures this to make sure each side of an optical span is assigned the correct complementary role rather than both ends configured identically. For example, one end of a DWDM span would be set to head-end while the far end is set to tail-end so the two plugs establish the link correctly.