This reports the frame encapsulation this physical FC port is actually using on the wire, i.e. whether it's carrying native Fibre Channel framing or FC encapsulated over another transport. For this specific port, the encapsulation in use determines what kind of cabling, transceiver, and far-end device are compatible with it, so a mismatch here is a common root cause when a port won't come up despite looking correctly cabled. For example, a port configured for native FC encapsulation will never link up against a switch port expecting FCoE framing, even over otherwise-compatible optics.