A similar device is used to create a list of notation for a technical (mathematical) work. Place a <notation> element as close as possible to the place where notation is first introduced. If you use the <definition> tag for your definitions, then this is a very natural place to also introduce notation. Inside of <notation> use the <usage> tag to include a short example of the notation in use. This will be treated as mathematics, so imagine that it will be wrapped in an <m> tag and use syntax. The <description> tag should contain a very short description in words of what the notation is for. So “center of a group” would be a good description to accompany the usage “$Z(G)\text{.}$”