Flank protection is one of the most important safety conditions which is realized by train routes controlled by signal boxes. As the name implies, a flank protection switch avoid collisions of trains to each other from the side. This dismissive position of the switch is normally for safe routes. For all neighbouring switches the dismissive position is a basic requirement of a route.
But not always this requirement can be implemented in logical route requirement. There are some exceptions. If there are two routes for example. When both of them need the dismissive but different position of the same flank protection switch. If switches can´t ensure flank protection, then the function has to ensured by a signal or a trackside derailer. Both the rule and the exception are explained in this clip.