B737 FLT CREW IS QUESTIONED REGARDING ALT ASSIGNMENT DSNDING INTO ORD ON ZAU FREQ.

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PROCEEDING DIRECT TO ORD; TALKING TO ZAU ON 125.57; FL370; CAPT WAS THE PNF; FO WAS THE PF. CLRED 'PLT'S DISCRETION' TO FL330. LATER CLRED 'DSND NOW' TO FL350. AS WE APCHED FL350; CLRED TO CONTACT ZAU 124.55. PASSING FL343; CHKED IN; DSNDING TO FL330. ADVISED BY CTLR TO CLB TO FL350. I RPTED LEVEL AT FL350 AND REQUESTED CTLR CHK WITH PREVIOUS CTLR ON 125.57 TO CONFIRM WE WERE CLRED 'PLT'S DISCRETION' TO FL330. CTLR SAID PREVIOUS CTLR DENIED ISSUING 'PLT'S DISCRETION' TO FL330. SHE REQUESTED WE CALL UPON LNDG. ARRIVED AT GATE AT XB15. AT XB30 SPOKE WITH MR X; THE SHIFT QUALITY ASSURANCE MGR; AND RELAYED THE ABOVE EVENTS. I ASKED HIM TO LISTEN TO THE TAPES AND ADVISED I WOULD CALL HIM BACK. AT XC30 MR X ADVISED ME HE HAD SPOKEN WITH THE 125.57 CTLR AND THIS CTLR AGAIN DENIED ISSUING THE 'PLT'S DISCRETION' TO FL330. THE TAPES PROVED THAT WE WERE INDEED CLRED TO FL330 PLT'S DISCRETION. I CALLED BACK AT XD00 AFTER MR X HAD LISTENED TO THE 124.55 TAPES. HE ADVISED ME WE HAD COMPLIED WITH ALL ISSUED CLRNCS. MR X ADVISED ME THAT MY RADAR TAG HAD BEEN LEFT AT FL350; NOT FL330 FOR THE HDOF; AS IT SHOULD HAVE BEEN; BASED UPON ISSUED CLRNCS. HE ADVISED ME THAT THIS WAS AN ATC ISSUE; AND STATEMENTS WERE BEING TAKEN FROM BOTH CTLRS. NO ACTION TO BE TAKEN TOWARDS CREW. HUMAN FACTORS ISSUES: EVENT HAPPENED AT XA45. NO TA'S OR RA'S. SAFETY NOT COMPROMISED.