Suppose one UE is capable to register with EUTRA as well as 2G/3G. In case of Idle mode signalling reduction is not used then if UE moving from EUTRAN to 2G/3G network or other way, UE needs to perform TAU/RAU procedure to the correponding network.
Idle mode signalling reduction is used to reduce the frequency of TAU /RAU when UE moving within EUTRAN/2G/3G network.
