Indicates the priority of the route <ul> <li>when there is only one route from entry to exit, priority=0</li> <li>when there are multiple routes from entry to exit, the lowest number has highest priority. Alternative routes are numbered 1..n</li> </ul> This allows a control system to attribute priorities to route setting. Whether or not the control system auto-selects lower priority routes when no higher priority route is available, depends on national rules and regulations.