This read-only ClockSourceIndex identifies the index of the network clock source actively supplying clock within this device, and per the description, when used as an index into cwnAtmSourceTable, if that entry's cwnAtmSourceBestClockSource is true, the indicated source is the root of a clock distribution tree. An admin follows this index to determine exactly which configured clock source this device is currently deriving timing from among potentially several candidates. Cross-referencing this value against cwnAtmSourceTable lets an admin confirm whether this device is itself acting as the root of the network's clock tree or just consuming clock from elsewhere.