From the maanual - "Extend Route:Extending is possible only when it is obvious, what should be joined with the current
route (current = visible in the Dialog). Eligible are all visible route points shared with, or lying nearby to the current
route's endpoint. If there is exactly 1 such point, then it is considered obvious, and the Extend button is enabled. If there are more, some should be hidden before proceeding, until just 1 remains.
Routes are extended forward, based on the geographic location, next route point must be very near, as when mouse-extending the route. If the next route point is to far away for extending, just create the joining leg and extend twice."