B757 FREIGHTER CAPT DELAYS DSCNT TO FL280 FROM FL390 AFTER CTLR HAD STATED 'NO DISCRETION.' FO HESITATED TO CALL ATTN TO THIS DELAYED DSCNT TO THE CAPT'S ATTN. NEXT AREA CTLR; ZID; GAVE THEM A REMINDER AND A NEW CLRNC; AND THEN SAID; 'CALL ME.'

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ON AUG/XA/97 AT APPROX AB01; FO AND MYSELF WERE OPERATING FLT; A B757; FROM ABY TO SDF. WE WERE IN CRUISE FLT AT AN ALT OF FL390 IN PCA CTLED BY ZTL. THE AUTOPLT WAS ENGAGED AND THE FMC INDICATED APPROX 2 MINS TO A VNAV PATH CAPTURE WHICH WOULD MAINTAIN AN APCH DSCNT. THE ACFT WOULD THEN CROSS AN ARR RESTR PROGRAMMED IN THE FMC FOR THE LOUISVILLE STANDIFORD ARPT. I REQUESTED THE FO TO OBTAIN A CLRNC FOR A LOWER ALT FROM ZTL WHICH WOULD ALLOW THE ACFT TO DSND NORMALLY WITHOUT FURTHER FLT PATH MODIFICATION. I OBSERVED THE FO PLACE FL280 IN THE ALT WINDOW AND SAY CLRED TO FL280 'NO DISCRETION.' I THEN REPEATED WHAT THE FO STATED TO ME AND BELIEVED THAT WE WERE CLRED TO DSND TO FL280 AT OUR DISCRETION. SINCE THE ACFT WOULD START THE PROGRAMMED DSCNT PATH IN ABOUT 6 MI AND I DID NOT HEAR DSND AND MAINTAIN FL280 NOW; CRUISE FLT AT FL390 WAS MAINTAINED. AT THAT POINT THE FO MADE NO INDICATION THAT THERE WAS ANY NON COMPLIANCE OCCURRING WITH THE FLT CLRNC RECEIVED. THE FO CHKED ON WITH ZID CTLR WHO IMMEDIATELY STATED 'I NEED FOR YOU TO DSND NOW. I STILL SHOW YOU AT FL390 SIR. WHAT ALT ARE YOU SHOWING?' THE FO STATED THAT WE ARE DSNDING AND WE ARE OUT OF FL385. DURING THE EXCHANGE AN IMMEDIATE DSCNT WAS INITIATED. THE CTLR GAVE US A TURN TO HDG WHILE WE WERE DSNDING AND THEN ANOTHER TURN BACK INTO THE TFC FLOW FOR AN ARR INTO LOUISVILLE. DSNDING THROUGH APPROX FL280 WE WERE REQUESTED TO CONTACT ZID WHEN ON THE GND AND GIVEN A PHONE NUMBER TO BE CALLED. THE FLT CONTINUED WITHOUT ANY FURTHER DEVS AND WE LANDED SAFELY AT THE LOUISVILLE STANDIFORD ARPT. ONCE ON THE GND I CALLED ZID AS REQUESTED AT APPROX AB54 AND LEFT A MESSAGE VIA THE ANSWERING MACHINE WITH THE REQUESTED INFO. I THEN CALLED AGAIN AROUND AI45 AND SPOKE WITH A REPRESENTATIVE AND ADVISED HIM OF THE ABOVE LISTED INFO AS REQUESTED. HE STATED THAT THERE HAD BEEN NO 'TFC SEPARATION' PROB AT THE TIME OF THE OCCURRENCE AND THAT THE TERM 'NO DISCRETION' WAS POSSIBLY CONFUSING AND SHOULD BE QUESTIONED IN THE FUTURE. A DEBRIEF WAS CONDUCTED WITH FO AND WE BOTH CONCLUDED THAT THE CONFUSION AROSE FROM THE INTERP OF THE TERM GIVEN 'NO DISCRETION.'