In 5G system, an UE can have 3GPP and non-3GPP connections simultaneously. When an UE moves to MM-IDLE state for a connection (3GPP/non-3GPP) while it stays in MM-CONNECTED for another connection(3GPP/non-3GPP), network uses the notification procedure to notify UE through the connected network(3GPP/non-3GPP) to re-activate the PDUs session belonging to other idle connection(3GPP/non-3GPP).
The important point here is re-activation of PDUs happen through 3GPP access. For example, if an UE is in MM-IDLE state for non-3GPP connection, network notifies UE to reactivate the PDU sessions belonging to non-3GPP through 3GPP access.
Paging procedure comes into picture when UE is IDLE state for both type of connection and there is no other way to reach the UE.