What is ISUP protocol? Why CPG message is required in ISUP protocol?

CPG message that Call is in Progress.Lets have a example considering a call forwarding scenario.As soon as switch receives Setup, it checks that the Called_Party has forwarded option activated and so tries to forward the call. Meanwhile, Switch sends CPG message towards originator saying that Call is in Progress.

 CPG message is sent by the switch towards originator.- When the switch receives the setup, the call is forwarded.- When the message is sent it is signaled that the call is progress.- During the setup or active phase of a call, a message is sent.- This signifies the relayed to the originating has occurred.