This read-only integer from 1 to 6 both indexes this row in the NTP server table and defines the order in which the Common Controller tries each configured time server, starting with index 1. It lets an administrator understand and predict the failover order used for NTP time sync if the primary server becomes unreachable. For example, an operator troubleshooting inconsistent clock sync could check whether index 1's server is offline, forcing the CC to fall back to index 2's server.