MD80 ACFT. FLC; GIVEN XING CLRNC; MISUNDERSTOOD IT AND DIDN'T COMMENCE DSCNT UNTIL CTLR INTERVENED AND QUESTIONED THEIR ABILITY TO MAKE THE XING RESTR. CTLR VECTORED ACFT TO AVOID CONFLICT. RPTR CAPT CONTACTED CTR AFTER LNDG TO CHK THE CLRNC.

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OUR CLRNC WAS: DIRECT TO THE IRQ VOR THEN THE UNARM ARR TO CLT. ALSO; WE WERE GIVEN A DSCNT CLRNC TO CROSS 15 MI S OF IRQ AT FL240. HOWEVER; I UNDERSTOOD AND THOUGHT I READ BACK; 'CROSS 20 MI N OF IRQ AT FL240.' THIS IS SHOWN ON THE UNARM ARR AS LUKES INTXN AND IS GIVEN TO US QUITE FREQUENTLY. THE FO AND PF ASKED ME; 'DID HE SAY 20 MI N OF IRQ AT FL240?' I SAID; 'YES; THAT'S CORRECT.' LATER; WHEN WE WERE IN OUR DSCNT (APPROX FL260 AND 18 MI N OF IRQ) THE CTLR ASKED US IF WE WOULD BE ABLE TO MAKE OUR RESTR OF 15 MI N OF IRQ AT FL240. I PAUSED; BECAUSE I DID NOT REMEMBER THAT CLRNC. THE FO SAID; 'DIDN'T HE SAY 20 MI N AND FL240?' I SAID; 'I THOUGHT SO.' THE CTLR THEN GAVE US A HDG TO AVOID MINIMUM SEPARATION WITH ANOTHER AIRLINER AT FL250. LATER; I CALLED ZTL TO FIND OUT WHAT OUR REAL CLRNC WAS AND FOUND OUT THE TAPES REVEALED 'CROSS 15 MI S OF IRQ AT FL240.' THE SUPVR ADVISED ME THAT NO MINIMUM SEPARATION STANDARDS HAD BEEN COMPROMISED. I BELIEVE THE FO AND I BOTH EXPECTED TO HEAR THE CLRNC '20 MI N OF IRQ AT FL240.' THEREFORE; WHEN THE ALT AND VOR WERE IN THE SAME CLRNC; WE AUTOMATICALLY WENT FOR THE CLRNC WE EXPECTED. (ALSO; WX AND FATIGUE WERE FACTORS.)