This flag is found at the MSC VLR (Visitor Location Register) on either a GSM or UMTS mobile network. The flag indicates that a mobile 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 fails to respond to paging messages and therefore determined to be unreachable. It is typically set after no response to two or three paging cycles, each paging cycle may contain two or three paging attempts.

MNRR-MSC Flag from 3GPP TS 23.040:

The Mobile-Station-Not-Reachable-via-the-MSC-Reason (MNRR-MSC) within the HLR stores the reason for the MS being absent when an attempt to deliver a short message to an MS fails at the MSC with the cause Absent Subscriber. The HLR updates the MNRR-MSC with the reason for absence when an absent subscriber diagnostic information is received from the SMS-GMSC and the MNRF is set. The HLR clears the MNRR-MSC when the MNRF is cleared. If the MNRF is set due to a failure at the MSC with cause Absent Subscriber and information pertaining to the absence of
the MS is not available from the SMS-GMSC, the MNRR-MSC shall remain in a cleared state. The MNRR-MSC shall either be in a cleared state or contain one of the following reasons:

No Paging Response via the MSC
IMSI Detached.