A 5G UE moves to RRC inactive state, when it receives RRC Release message with suspendconfig parameter structure, which provides the UE with I-RNTI, which is used to address UE during Inactive state in paging message, a UE also uses I-RNTI in RRC Resume request message.
The RRC Inactive state allows the UE to return to RRC Connected and start transferring application data or signaling messages with minimal latency. Signaling load is reduced relative lo the RRC Idle to RRC Connected transition because the UE context is already established. The RRC Inactive state allows the UE to reduce battery power consumption relative to RRC Connected state.
This is mainly used for MTC, UE which need to frequent change states.
Note - LTE also allows RRC inactive state from Rel 14.
3GPP References: TS 38.331
