This would apply to Verizon (now Fairpoint) territories here in Maine. Not sure if this is company wide.

ARDU - dry loop with no Verizon services on it
TXNU - lineshare DSL + Voice, possibly mixed ILEC circuit with ILEC Voice and CLEC DSL service on it
ARDA - Verizon only circuit, voice + DSL

Where would I find this circuit identification? Possibly written in the NID or on a circuit identification tag in the NID on the wires.