1) UE should know whether it can support ISR or not and it tells to network.
2) UE should know whether ISR is activated after requesting or not from network.
If UE supports ISR then it request through these two messages to network to activate ISR
* Routing area update request / Attach request Message ---> MS network capability ----> ISR flag
ISR flag :- UE tells about whether it supports ISR or not.
After applying for request if network accepted and activated the UE should be informed about this ISR activation.
* About the activation UE knows when the ISR activation status status comes in
* TAU Accept and RAU Accept ----> Update result flags
For more u can refer 24.004 , 24.008