This is an Integer32 reading of the received optical power that the SFP transceiver on a SONET/STM optical line reported, retrieved directly from the SFP's own diagnostic registers. An admin watches this because a receive-power reading that drifts low or high over time usually means a dirtying/degrading fiber connector, a bending fiber, or an aging laser on the far end, letting the fault be caught before the OC-n/STM line actually drops. For example, if ocnStmHistoryLineOPR on a given OC-48 port slides from -8 dBm down toward -20 dBm across successive history bins, that trend alone justifies scheduling a fiber cleaning or SFP swap.