NSSCHECK is a must if you are about to upgrade a NetWare 5.x server to NetWare 6.x. Although we have seen issues only in volumes with multiple segments, one should check all NSS volumes before upgrading NetWare 5.x.
NSSCHECK.NLM is a utility to check the format of the MAL information stored on the disk. It is completely different than validating a volume. It only checks the MAL segment information to see if it is valid for an upgrade to work correctly.