A DC10-30 FREIGHTER CLBS TOWARDS FL250 WHEN THE FLT HAD BEEN ASSIGNED A '2-5-0' BY ZBW CTLR. CTLR MEANT KTS; BUT THE CREW THOUGHT IT WAS A FLT LEVEL.

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DURING CLBOUT FROM EWR; WE HAD RECEIVED SEVERAL STEP-CLB AND CHANGE OF HDG CLRNCS FROM ATC. WE WERE IN A CLB TO FL210. AFTER SWITCHING TO ZBW; WE WERE GIVEN A CLRNC OF 'TURN R 040 DEGS; MAINTAIN 2-5-0.' THE FO; RESPONDED TO THE CTLR; 'HDG 040; CLB TO 2-5-0.' AT ABOUT FL237; DURING THE CLB; THE CTLR TOLD US TO DSND TO FL210. HE SAID WE HAD NOT BEEN CLRED TO FL250. HE THEN ASKED IF WE WERE MAINTAINING 250 KTS. WE WERE NOT; AS WE HAD NOT BEEN ISSUED THIS RESTR. WE WERE ASKED TO SLOW TO 250 KTS; AND WE COMPLIED WITH ALL FURTHER INSTRUCTIONS. I BELIEVE THIS OCCURRENCE COULD HAVE BEEN AVOIDED HAD THE CTLR USED CLRER; STANDARD RADIO PHRASEOLOGY. THERE MAY ALSO HAVE BEEN SOME CONFUSION TO HIM AS TO OUR CLRNC ON THE HDOF FROM THE PREVIOUS SECTOR. TO MY KNOWLEDGE; NO TFC CONFLICT WAS INVOLVED. SUPPLEMENTAL INFO FROM ACN 428344: CTLR USED 2-5-0 INSTEAD OF FL250. I READ BACK CLBING TO 2-5-0 AND HE DID NOT CORRECT ME. USING 2-5-0 INDICATES AN ALT TO ME AND 250 KTS MEANS AIRSPD. SUPPLEMENTAL INFO FROM ACN 428184: AS SO (FE) I WAS CONTACTING BOS ATIS AND BOS RAMP AT THIS TIME. WHEN I GOT OFF THE RADIO WITH BOS RAMP THE FRONT ENDERS WERE DSNDING BACK DOWN TO FL210.