AN AIRBUS A320 FLC DSNDED BELOW THE ASSIGNED ALT RATHER THAN MAINTAIN THE ASSIGNED AIRSPD NEAR MSP. AT THE HDOF POINT THE ACFT WAS ABOVE THAT SECTOR'S ALT.

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I HAD LEFT THE FLT DECK TO USE BATHROOM; WE WERE APCHING MSP ARPT AT CRUISE FL290. UPON RETURN FROM BATHROOM; THE CTR CALLED AND ASKED OUR ALT. I RPTED WE WERE JUST LEVELING AT FL270. THE CTLR STATED HE HAD NOT AUTHORIZED FL270. THE FO THEN TOOK RADIO AND SAID TO THE CTLR THAT ON THE FREQ CHANGE HE HAD BEEN INSTRUCTED TO DSND TO FL270. THE CTLR THEN STATED YOUR CLRNC WAS TO MAINTAIN 270 KTS. THE FO THEN STATED TO THE CTLR THAT HIS CLRNC WAS TO MAINTAIN FL270; AT WHICH TIME HE HAD STATED MAINTAIN FL270 LEAVING FL290 FOR FL270. WE THEN GOT ANOTHER FREQ CHANGE; AT WHICH I STATED FL270; 270 KTS. THIS CTLR QUESTIONED MY ALT. I ASKED HIM IF THERE WAS A PROB. HE SAID 'STAND BY.' HE THEN CAME BACK AND SAID NO PROB; THE PREVIOUS CTLR SHOULD HAVE DSNDED US TO FL230 AS HIS AIRSPACE STARTED AT FL230. IT'S A CTR COORD THING; NO PROB. TO SOLVE THESE KINDS OF MISCOMS; WE NEED TO DISTINGUISH BTWN FLT LEVEL AND MAINTAIN SPD -- NOT JUST MAINTAIN 270. SUPPLEMENTAL INFO FROM ACN 444150: I; FO; CHKED IN WITH ZMP AT FL290. HE CLRED ME TO FL270 AND I ACKNOWLEDGED AND READ BACK FL270.