This enumerated value selects whether this network element sends the older 1-bit RDI-V (Remote Defect Indication, VT-level) signal for backward compatibility with older equipment, or the newer 3-bit enhanced RDI-V that can convey more specific defect information; the default is onebit. An admin cares because using the enhanced 3-bit format only works if the remote end also understands it, so this setting must match capabilities on both sides of a VT-level tributary link. For example, connecting to legacy SONET equipment that only recognizes 1-bit RDI-V would require leaving this at the onebit default rather than upgrading to threebit.