This selects whether this DS1 port's performance-monitoring threshold definitions follow RFC 1406 or ITU-T G.826. The two standards define errored-second and severely-errored-second thresholds somewhat differently, so this setting on this specific port determines which numeric criteria actually trigger the PM threshold-crossing alerts reported elsewhere, like pportPMTcaId, which matters for matching whichever standard a carrier SLA or regulatory requirement specifies. A circuit under a contract that specifies ITU-T G.826 performance criteria would need this port set to itu-g826 rather than the RFC 1406 default so its threshold alerts align with the contracted standard.