MNRF (Mobile-Station-Not-Reachable-Flag) is a flag that is set at the HLR (Home Location Register) on either a GSM or UMTS mobile network. It indicates that a mobile station is not reachable and thus prevents any further and unnecessary signaling. For example, the flag would be set in the event that a mobile station failed to respond to a paging message.
The significance of MNRF is that it can reduce the waste of network resources and improve the efficiency of SMS delivery. By setting the MNRF, the network can avoid sending SMS messages to mobile stations that are not ready to receive them, and instead wait until they become reachable again. This can also prevent the congestion of the network and the SMSC (SMS Center).