Flight Crew Air Transport Pilot (ATP)
Human-Machine Interface; Communication Breakdown; Confusion
ATC Issue All Types; Deviation / Discrepancy - Procedural Clearance
General None Reported / Taken
Human Factors; Procedure
We were filed to RDU on the South Boston Four Arrival via ROA. The last ATC Controller prior to Approach Control cleared us direct to ALDAN (a fix on our filed route and the STAR). We understood the clearance to be 'direct ALDAN; comply with the arrival and arrival speeds.' When we were handed off Approach Control asked if we were flying the ALDAN ONE Arrival?  We answered 'no; we're on the South Boston Four' and he replied. 'OK; that explains it.' We never understood the Controller to say expect the ALDAN ONE or plan to fly the ALDAN ONE Arrival (a different arrival than originally filed). Both arrivals have the same early fixes at ROA; ALDAN at 12;000 FT and BILLA. After BILLA the Approach Controller gave us heading and altitude vector which we were expecting while flying the South Boston arrival. The vector was given to comply more with the ALDAN ONE arrival the Controller thought we were flying. We never changed the arrival information in our FMS (flight plan) and complied with all crossings and speed restrictions on the SOUTH BOSTON FOUR arrival landing northeast on Runway 5L at RDU. There needs to be better clarification between ATC and pilots as to which arrival to fly into RDU being that both arrivals have the same fixes starting at ROA on the two arrivals in question.
Confusion reigned when ATC cleared an E-170 flight crew direct to ALDAN; a fix on both their filed South Boston STAR into RDU and the 'ALDAN' STAR--which includes several common fixes. At no time were they cleared 'via' either STAR. The flight was eventually vectored for an approach and the confusion never satisfactorily resolved.