s that an annex is normative (meaning it is part of the specification) and an appendix is in- formative (only for information and educational purposes). To indicate the importance of material in an annex, the annex has a subheading that states "This annex forms an integral part of this Recommendation." To claim compliance with a recommendation, an implementor must address the annexes but not ne- cessarily the appendixes. Despite this, the annexes to some recommendations may be an optional component. On the other hand, the appendix is for any information that is considered as merely il- lustrative or supplementary to the recommendation. This description of those recommendations is current to those editions published resulting from the ITU-T (virtual) Plenary Meeting in September 2020. As outlined in the section on the development process, the consent happens at that plenary meeting, but the formal process of approval and public- ation takes several months. 8.1.1 Physical and TDM Versus Packet Recommendations The ITU has developed many recommendations related to timing in the areas of SDH, SONET, and SyncE. For the purposes of this book, the treatment of these recommendations is split into physical (non-packet) and packet-based timing, but of course, there is overlap. Obviously, there is no one ideal method for categorization, but this layout should make it as clear as possible. Just remember that the boundaries between the categories can be somewhat blurred. For example, there is merging of aspects of the packet and physical approach to frequency syn- chronization in some of the recommendations such as G.8261. In addition, the implementation of packet-based phase and time synchronization may be implemented in combination with a physical frequency distribution over a packet network, such as SyncE. Table 8-2 illustrates the method used to group and categorize the recommendations for this chapter. Table 8-2 Layout and Categories of the Recommendations for This Chapter Methods Frequency Phase and Time Physical and TDM