This read-only attribute reports which clock is actually in use for synchronization right now - internalClk, priNetworkClk, or secNetworkClk - defaulting to internalClk. It's the readback that confirms the actual result of the hcxSyncClkConfigure setting and any OC3 card health/failover logic, letting an admin verify the system correctly switched to a secondary source if the primary failed. For example, after a primary OC3 card fails, an engineer would check hcxSyncClkSelect to confirm it correctly failed over to secNetworkClk rather than dropping all the way back to the free-running internalClk.