This reports which of the five possible timing sources (external A/B, port reference A/B, or the internal oscillator) the node is actually using right now for its secondary sync reference. Because this is the secondary (backup) reference, seeing it fall back to internal tells the admin the external or port timing feeds normally used have failed and the switch is running on its own free-running oscillator, which can drift and eventually cause slips. For example, an admin polling this OID after a WAN clock outage sees it return internal(5), confirming both external timing sources have failed and holdover has kicked in.