TURBOPROP CAPT MISINTERPED DSCNT CLRNC RESULTING IN OVERSHOOTING ASSIGNED INTERMEDIATE DSCNT ALT AND A CONFLICT WITH ANOTHER ACFT.

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E OF GIJ; ZAU TOLD ME TO DSND TO 13000 FT. SOMETIME DURING THE DSCNT; I WAS TOLD TO GO TO ZOB AND DID SO. AFTER CHKING IN WITH ZOB AND INFORMING HIM I WAS DSNDING TO 13000 FT; THE CTLR ASKED MY RATE OF DSCNT. I TOLD HIM IT WAS 2100 FPM AND HE TOLD ME TO 'MAINTAIN FIFTEEN.' I ANSWERED; 'ROGER; MAINTAIN FIFTEEN.' I THOUGHT HE MEANT 15000 FPM AND IT DIDN'T OCCUR TO ME THAT HE MIGHT MEAN ONE FIVE THOUSAND. I GUESS IT WAS BECAUSE HE HAD JUST ASKED MY RATE OF DSCNT AND THE USE OF FIFTEEN INSTEAD OF ONE-FIVE. I SHOULD HAVE ASKED FOR VERIFICATION. I ASSUMED THE CHANGE IN RATE OF DSCNT WAS FOR TFC AND; OF COURSE; THIS HAS OCCURRED MANY TIMES BEFORE. AT ABOUT 14200 FT; THE CTLR ASKED ME IF I RECEIVED THE ALT AND AT THIS POINT I DON'T RECALL HOW THE ALT WAS SAID AND BEFORE I COULD ANSWER HE GAVE ME A CLB TO 15000 FT WHICH I INITIATED THEN HE CHANGED THAT TO A DSCNT TO 11000 FT WHICH I BEGAN IMMEDIATELY. HE ALSO TOLD ME TO EXPEDITE. HE TOLD THE OTHER ACFT TO EXPEDITE TO FL190. I BELIEVE THE OTHER ACFT WAS AN ACR FLT; BUT I DON'T KNOW THE TYPE. THE ACR CALLED ME IN SIGHT; BUT I NEVER SAW HIM. THIS IS MY BEST RECOLLECTION OF THE EVENT. I NORMALLY VERIFY QUESTIONABLE CLRNCS WITH ATC; BUT IN THIS CASE MY PERCEPTION WAS A CHANGE OF RATE OF DSCNT INSTEAD OF ALT.