The revision adds B1T content, which also includes changing description of OIF assigned PT.

Recommendation ITU-T G.709.1 specifies common elements and signal structures used by various types of flexible optical transport network (FlexO) interfaces. Edition 4.0 is a major revision, with a new FlexOPy path networking layer added. The overheads have been renamed based on function instead of location in the frame structure. 1.6T Ethernet clients and associated mapping procedures have been added. Finally, a new FlexO-xo rates is introduced for OTN applications.