A Mapped Area is a collection of Official GPS Routes for an area. Look at the Phoenix Preserves near Piestewa Peak for example. Here the most popular inner city trail is surrounded by lesser known trails. Reviewing it here makes it easy to form your own loops.
If you are interested in creating a new Mapped Area contact the webmaster with a comma separated list of description IDs. That is the ZTN= figure listed in the URL of description pages. Maps under 200 routes are best.