This read-only Integer32 is the 32-bit sum of LS checksums of all type-7, NSSA External, LSAs in this area's link-state database, per its description, giving a fast way to detect a change in the NSSA's locally-originated external route set. Comparing this value across all routers within the same NSSA is a quick consistency check that they all hold the same set of type-7 LSAs, without needing to diff each externally-redistributed route individually. An engineer troubleshooting an NSSA where some routers seem to be missing certain external routes would compare this checksum sum across the area's routers to localize which router has a stale or incomplete view.