The New Zealand Railways Magazine, Volume 2, Issue 7 (November 1, 1927)
A shunting signal consists of a round target either on the ground or elevated on a post. One shunting signal may indicate several routes, and appropriate locking is provided for each route. The intricacy of the locking plate is mainly due to the necessity for providing the maximum freedom of shunting movements consistent with safety, and alternative combinations of locking must be provided which become effective according to the routes set and the shunting signals used together. Such special locking combinations are collectively known as “permissive” locking.