This amendment contains extensions to the fifth edition (06/2016) of ITU-T Recommendation G.709/Y.1331, related to the: - Addition of interconnection of Ethernet UNI and FlexE Group UNI in two administrative domains (7.5, Appendix XIV). - Addition of 50GBASE-R mapping into ODUflex (2, 17.13.3, Annex J). - Update of FlexE terminology and calendar slot descriptions with FlexE 2.0 IA (2, 17.11, 17.12). - Redefine the OP overhead bit in bit 1 of JC6 as a RES overhead (20.4, 20.4.3.1, 20.5). - Replacement of references to IEEE Std. 802.3-2015, 802.3by and 802.3bs by a reference to IEEE Std 802.3-2018 (2, 17.13.1, 17.13.2, Annex J, Annex K). - Addition of ODU4 and ODUflex(25G/50G/200G/400GBASE-R) rows to Table 7-9 (7.4). - Addition of 25G, 50G, 200G and 400GBASE-R rows to Table IX.1 (Appendix IX). - Addition of ODUflex(25G/50GGBASE-R, FC-1600/3200) rows to Table X.1 (Appendix X).