The <idx> tags denote an index entry. These should be placed within the element that they describe. By this we mean that an <idx> element can be placed within a <theorem> to refer to just that theorem, or it might be placed within a <subsection> to refer to that subsection. In this way, electronic versions of your work can have an index that is more informative than a traditional index that uses just page numbers. Note that the text contained within the <idx> tags does not actually appear in the article—it only serves to mark the location the index entry points to. You can have several levels of headings by structuring your <idx> element with up to three <h> tags. Alternatively, after the first use of an <h> element, you can use <see> or <seealso> to denote a “see” or “see also” in the index entry, respectively.
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 “$Z(G)\text{.}$”