FLC OF A CANADAIR; CL65 CRJ; FOLLOWED THEIR FILED FLT PLAN RATHER THAN THE FLT PLAN CLRNC GIVEN PRIOR TO DEP RESULTING IN ATC VECTORS TO AVOID OTHER TFC.

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THIS WAS THE LAST DAY OF 4 STRAIGHT DAYS OF FLYING. THE PREVIOUS DAY WE FLEW OVER 7 1/2 HRS INVOLVING GND DELAYS; INFLT DEVS AND HOLDING DUE TO TSTMS. TODAY WE WERE SCHEDULED FOR ALMOST 6 HRS OF FLYING. THIS WAS THE SECOND FLT OF THE DAY FROM BNA TO IAD. THIS FLT WAS FILED WITH THE FOLLOWING RTE: BNA...HVQ...JASEN2.IAD. WHEN CLRNC DELIVERY ISSUED OUR CLRNC IT WAS: 'CLRED THE NASHVILLE 8 DEP; AS FILED; MAINTAIN 5000 FT; DEP FREQ IS 118.4; SQUAWK XXXX.' THIS IS WHAT WE INPUTTED INTO THE FMS COMPUTER AND FLEW. THERE IS A SECOND RTE THAT WE OFTEN FLY; IT IS: BNA...SWAPP.J42.BKW.JASEN2.IAD. ONCE WE WERE AIRBORNE; DEP ISSUED SEVERAL VECTOR HEADINGS. THE FINAL ONE WAS TO 'MAINTAIN...TO JOIN...' BECAUSE OF FATIGUE; I CANNOT RECALL IF HE SAID '...TO JOIN THE JET RTE' OR '...TO JOIN YOUR RTE.' I ANSWERED BACK VERBATIM WHAT HE STATED; AND SET THE HEADING INTO THE FMS COMPUTER. THE ISSUED HEADING WAS A 30 DEG INTERCEPT TO OUR FILED ROUTING AND LOOKED LIKE A GOOD INTERCEPT HEADING. THE FMS DOES NOT DISTINGUISH BTWN DIRECT ROUTING OR JET ROUTES. WE WERE THEN HANDED OFF TO CTR CTL. CTR ACKNOWLEDGED OUR INITIAL CALL AND SAID NOTHING OF OUR POS OR HEADING. APPROX 20 MINS LATER CTR INQUIRED IF WE WERE ON THE JET RTE. I WAS CONFUSED BY HIS QUESTION; BUT REPLIED THAT WE FILED VIA DIRECT ROUTING AND CLRNC DELIVERY HAD STATED 'CLRED AS FILED.' CTR THEN STATED THAT WE SHOULD HAVE BEEN ON THE JET ROUTING; HE GAVE US VECTORS SO WE DID NOT CONFLICT WITH OTHER TFC; AND STATED HE WOULD HAVE TO LOOK INTO THIS. AT NO TIME WERE THERE ANY CONFLICTS OR OTHER DANGERS TO ACFT. WE CONTINUED WITH OUR DIRECT ROUTING WITH NO OTHER PROBS.