E-tags are “evaluate tags” as opposed to “session tags” (S-tags). In practice, E-tags are very, very similar to S-tags. The main difference is that sometimes if you nest an S-tag instead of another S-tag, it won't work. When that happens, set the outer S-tag to an E-tag, e.g.:

This won't work:
[[S36:top_participants_list,[[S80:trID]],SCROLLING,SUM,10,Top Walkers]]

But this will:
[[E36:top_participants_list,[[S80:trID]],SCROLLING,SUM,10,Top Walkers]]