A "composite trail" is a trail with multiple segments, such as the <a href=RoutexPg.asp?RoutexId=8>North Boundary Trail</a>. The general name for composite trails and composite roads is "composite routes". The word "route" implies that it uses multiple segments. If you look at the North Boundary Trail page, you see multiple "Trail Segments". The concept of composite "routes" is well established in the highway world. Eg: The famous "Route 66" consists of many separate highway segments as you go from state to state. Similarly the "Yellowhead route" is made up of several highways.
How it works:
There is one database record for the entire route, and then separate database records for each segment. The segments are just regular "Roadx" records. The route has its own set of waypoints, (which often duplicate the segment waypoints) so that the route can have waypoints for high country segments where there is no actual underlying trail record.
There are only a couple of dozen composite routes in the system. Here is <a href=http://bivouac.com/RoutexLister.asp>a list of composite routes</a>.