CAPT OF A DORNIER; DO328; QUESTIONS THE LEGAL POS FOR CARRYING A COMPANY RESERVE FLT ATTENDANT TO ANOTHER BASE IN COCKPIT JUMP SEAT AT THE REQUEST OF THE COMPANY. THERE WAS NO OPEN SEAT AVAILABLE IN THE PAX CABIN.

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FLT BOARDED PAX; 31 PEOPLE; LEFT GATE AND DURING TAXI TO ACTIVE RWY; WAS TOLD TO CONTACT COMPANY. COMPANY SAID TO RETURN TO GATE AND PICK UP A RESERVE FLT ATTENDANT WHO NEEDED TO BE IN TRI FOR FLT IN MORNING. AFTER ARRIVING AT GATE I CALLED COMPANY TO LET THEM KNOW THAT OUR PLANE HAD ONLY 1 SEAT AVAILABLE AND THAT SEAT WAS INOP DUE TO AN MEL; BUT IF THEY COULD GET PERMISSION FOR THE FLT ATTENDANT TO RIDE IN JUMP SEAT; WE COULD TAKE HER WITHOUT HAVING TO REMOVE A PAYING PAX. THE DISPATCHER TOLD ME TO WAIT WHILE THEY CHKED TO SEE IF WE COULD PUT THE FLT ATTENDANT IN THE JUMP SEAT. AFTER A MIN THEY CALLED ME BACK AND GAVE ME PERMISSION TO TAKE THE FLT ATTENDANT IN THE JUMP SEAT. QUESTIONS HAVE ARISEN AS TO WHETHER WE VIOLATED AN FAR BY CARRYING THE FLT ATTENDANT IN JUMP SEAT; WITHOUT HAVING AN OPERABLE SEAT IN REAR OF ACFT THAT THE FLT ATTENDANT COULD OCCUPY IN CASE OF AN EMER. ALSO OUR MANUAL SAYS WE CAN'T HAVE ANYONE IN JUMP SEAT THAT DOESN'T HOLD AN AIRMAN CERTIFICATE. THAT'S WHY I HAD DISPATCH GET PERMISSION FOR ME TO DO THIS. THEY MAY HAVE ACTED IN ERROR.