The scenarios of these two are different. SystermInfomodification is used during paging and InfoValueTag is indicated in periodic SIB1 transmission itself to tell if there is any change in System Information.
The RRC Paging message is used to inform UEs in RRC_IDLE and UEs in RRC_CONNECTED about a system information change. If the UE is in RRC_CONNECTED or is not configured to use a DRX cycle longer than the modification period in RRC_IDLE, and receives a Paging message including the systemInfoModification , it knows that the system information will change at the next modification period boundary. A UE in RRC_IDLE that is configured to use a DRX cycle longer than the modification period, and receives in an eDRX acquisition period at least one Paging message including the systemInfoModification-eDRX , shall acquire the updated system information at the next eDRX acquisition period boundary.
SystemInformationBlockType1 (or MasterInformationBlock-NB in NB-IoT) includes a value tag systemInfoValueTag , that indicates if a change has occurred in the SI messages. UEs use systemInfoValueTag , e.g. upon return from out of coverage, to verify if the previously stored SI messages are still valid. Additionally, the UE considers stored system information to be invalid after 3 hours from the moment it was successfully confirmed as valid unless specified otherwise. BL UE or UE in CE mod considers stored system information to be invalid after 24 hours from the moment it was successfully confirmed as valid.
The UE using a DRX cycle shorter than or equal to the modification period verifies that stored system information remains valid by either checking systemInfoValueTag in SystemInformationBlockType1 (or MasterInformationBlock-NB in NB-IoT) after the modification period boundary, or attempting to find the systemInfoModification indication at least modificationPeriodCoeff times during the modification period in case no paging is received, in every modification period . If no paging message is received by the UE during a modification period, the UE may assume that no change of system information will occur at the next modification period boundary. If UE in RRC_CONNECTED, during a modification period, receives one paging message, it may deduce from the presence/ absence of systemInfoModification whether a change of system information other than ETWS information, CMAS information and EAB parameters will occur in the next modification period or not.
When the RRC_IDLE UE is configured with a DRX cycle that is longer than the modification period, and at least one modification period boundary has passed since the UE last verified validity of stored system information, the UE verifies that stored system information remains valid by checking the systemInfoValueTag before establishing or resuming an RRC connection.