From the Unix attributes tab of an Active Directory account or account template property sheet;
What does Use System Assigned UID mean?
It means that when adding the user to a NIS domain and there is no value for "uidNumber" provided, and so requesting to use the next available "uidNumber"
then the next available "uidNumber" is found as the attribute value "msSFU30MaxUidNumber"
under the "CN=<YOUR_DOMAIN>,CN=ypservers,CN=ypServ30,CN=RpcServices,CN=System" DN branch from your ADS.
Our ADS CCS plug-in agent gets this ADS msSFU30MaxUidNumber attribute value to set the uid number of the new account in this case.