This amendment contains extensions to the fifth edition (06/2016) of ITU-T Recommendation G.709/Y.1331, related to the: -Addition of paragraph that describes that G.798 complements G.709 (1) -Addition of references to Rec. G.8260 (2) -Addition of definitions and conventions that have been included in the past in G.870 (3, 5) -Deletion and addition of abbreviations (4) -Addition of a sentence that describes that 3R regeneration is used on both sides of an interface interconnecting user and provider administrations and two provider administrations (6) -Corrections to the STU, MOTU, SOTUm and MOTUm descriptions (8.1, 8.2, 8.3, 8.4) -Addition of a note describing a vendor specific mode in the GCC0 communication channel deployment (15.7.2.2) -Deletion of OTUCn OSMC (15, 15.7.1, 15.7.2.4, 15.7.2.4.1.2) -Deletion of reference to concatenation (15.9.2) -Aligning the OPUk/OPUflex clock performance requirement in case of BMP based client mapping with G.8251 and G.798 (17.2, 17.19, 17.12) -Clarify the 2-bit alignment when mapping 66-bit blocks into the OPUflex payload area (17.11, 17.12) -Changing the minimum number of FlexE signals in a FlexE aware mapping into an OPUflex (17.12) -Addition of 25GE client signal (2, 15.9.2.1.1, 17.13, 17.13.1) -Deletion of ODUCn HAO reference (20.4.3.1).