Train operations require signals other than main line signals to control movement of trains within yard for management of train operations. Such movements are of two types

a) Movements required for changing coaches, locomotives, wagons and maintenance activities

b) To take care of failures of the main signals

Movements as described in a)  are taken care of by signals referred to as shunt signals 

Movements as described in b) are taken care of by signals referred to as calling on signals/indicators.

Shunt signals are generally placed after the first stop signals and before the last stop signals at a station. This ensures that shunting can take place within the limits of a station and trains moving in form other stations are protected from conflicting with shunt movements which is an entirely local operation. Unlike main signals shunt signals can be cleared even when a track is occupied in the route of the shunt signal. 

Shunt signals are often placed on starter signals and just in front of the diverging junction facing the starter signals. A typical arrangement of shunt signals is at Figure 6. Shunt signals are represented as small circles in the figure.

Shunt signals can be independent or dependant. When a signal is co located with a main signal it is a dependant signal and may be only referred to as main signal no with S added at the end. Thus shunt signals on Signal S7 and S9 are S7S and S9S and are dependant signals. Shunt signal S102 is a independent  shunt signal.