This enumerated value selects how DS0 robbed-bit signalling is carried across a byte-synchronous T1 VT mapping: signallingTransferMode moves the robbed-bit signalling into the VT header's S bits and discards the framing bit, while clearMode transfers only the framing bit and leaves signalling bits intact in the VT payload; notApplicable applies when byte-sync mapping isn't used. An admin cares because choosing the wrong mode here can silently strip or corrupt channel-associated signalling (like on-hook/off-hook state) for voice circuits riding this T1 tributary. For example, a T1 carrying PBX trunk signalling that depends on robbed-bit signalling reaching the far end correctly would need signallingTransferMode rather than clearMode.