A DC9 CREW AT FL350 IS ASSIGNED FL340 AND QUESTIONS THE CTLR AS TO THE LEGALITY OF THE NON STANDARD ALT.

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AT XA15Z; WE WERE AT CRUISE (FL350) FROM ORD TO MSP AND UNDER CTL OF ZMP (FREQ 134.25). THE CTLR GAVE US A HEADING OF 330 DEGS (OR SOMETHING CLOSE TO THAT). SO WE FLEW NE OF THE ALO VOR TOWARDS MSP. THE VECTORS WERE FOR SPACING INTO MSP. AT APPROX XA57Z; THE CTLR TOLD US TO DSND TO FL340. I ASKED THE CTLR IF THAT WAS A LEGAL ALT AND THE RESPONSE WAS SOMETHING LIKE 'EXPECT A LOWER ALT AFTER YOU DSND' FOLLOWED BY 'YOU HAVE TFC AND YOU WOULD HAVE TO TURN 40 DEGS TO THE L.' I RESPONDED SAYING WE WOULD TAKE THE TURN. WE TURNED AS ASSIGNED THEN SOON AFTER; THE CTLR GAVE US A DSCNT TO FL320. I CONCLUDED THAT WE NEEDED TO DSND TO RESOLVE SOME CONFLICT AND DID BEGIN TO DSND. THE CTLR THEN ASSIGNED US FL330 OR FL310 WHICH WERE NORMAL ALTS IN THE AIRSPACE STRUCTURE. THIS MADE US FEEL A LITTLE MORE COMFORTABLE; AND THE REST OF THE FLT WENT NORMALLY. THE FOCUS OF THIS RPT IS MORE ON MY REACTION TO ALT ASSIGNMENTS OF FL340 AND FL320; WHICH; IN OVER 30 YRS OF FLYING; I UNDERSTOOD NOT TO BE LEGAL OR USABLE ALTS. WE WERE VERY UNCOMFORTABLE WITH THE UNUSUAL ALT CLRNCS.