in fossil there is no way to add a comment to a tag (to explain the concept behind this tag). 

i know there are values, but the value can change per checkin:

i.e.: <br>
checkin 53739292 tag foo=bar<br>
checkin 39203748 tag foo=bar2

i think it could be useful to have a comment which explains a tag. <br>
this comment could also be displayed in the taglist (http://www.fossil-scm.org/index.html/taglist)