See
Figure 1. A trail is a specific path which is recorded during your ride. While the trail is recording, the system stores ongoing location points along the ride. This is different than saving a destination or trip, which only store a number of locations rather than a specified path.
A trail can be used to repeat the ride along the same path in the future or to route back to the start of the trail using the same path.
A trail can be recorded at any time. A route does not need to be active to begin recording a trail.
Figure 1. Recording a Trail