This is the minimum SSM quality level a candidate timing reference must meet before the selection algorithm for this protection group will even consider using it, acting as a quality floor below which a reference is treated as unusable regardless of its configured priority. An admin tightening network timing accuracy requirements, for example for a mobile backhaul application, would raise this threshold to ensure the system never selects a degraded-quality source even temporarily. For example, setting this to a stratum level equivalent to primary reference clock quality would cause the system to reject any reference reporting a lower quality level, even if that reference is otherwise available.