." Appendix VI, "Clarification of the use of the acronym 'SEC'." This appendix clarifies the use of the term SEC to mean synchronous equipment clock as well as SDH equipment clock and how it relates to the quality level QL-SEC. This point is also made throughout this chapter. Appendix VII, "Use cases of mixing SSM and eSSM." G.781 defines a very important function for the frequency synchronization approach, which is de- fining the process to select the best available frequency source (in clause 5 in combination with An- nex A). See the section "Clock Selection Process" in "Chapter 6. Physical Frequency Synchroniza- tion", for an explanation of how this process works. Another important point in G.781 also deals with the new meaning of the acronym SEC, which is explained in Appendix VI and clause 3.6. Originally the term "SEC" was used to refer to an SDH equipment clock, but now the term has been changing (from about 2018) to mean synchronous equipment clock. There is also a note explaining this in the section on G.8261 later in the chapter. This new SEC is a generic term used to represent all the following: the SDH equipment clock (G.813), the Ethernet equipment clock (EEC in G.8262), and the OTN equipment clock (OEC in G.8262). However, some of the older, TDM-based recommendations (for example, G.813) still use SEC to refer to the specific SDH equipment clock. Link to recommendation: https://handle.itu.int/11.1002/1000/14240 G.810: Definitions and Terminology for Synchronization Networks ITU-T G.810 is quite an old recommendation, and it has hardly been touched for more than 20 years. It gives definitions and abbreviations used in the ITU-T timing and synchronization recom- mendations. From the scope of the recommendation: This Recommendation provides definitions and abbreviations used in timing and synchron- ization Recommendations. It also provides background information on the need to limit phase variation and the impairments on digital systems. This recommendation includes the following main topics and append