ALT BUST; ACCORDING TO ATC.

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WHILE CLBING AFTER OUR DEP WE WERE BEING VECTORED TOWARD OUR DEP FIX. HDG APPROX 240 TO 4000 FT. ON THAT HDG WE REQUESTED AND RECEIVED CLRNC FOR A 20 DEG TURN TO THE R TO AVOID WX. THE ATC REPLY WAS '20 DEGS R APPROVED; CLB MAINTAIN 12000 FT.' THE PNF REPLIED R 20 DEGS; MAINTAIN 12000 FT AND SET 12000 FT INTO THE ALT ALERTER CONFIRMED BY MYSELF. THE ALT WAS AN UNUSUAL ONE FOR OUR DEP; BUT BECAUSE OF OUR DEV I THOUGHT IT MIGHT HAVE BEEN NECESSARY FOR CONFLICTING ARR TFC. AFTER CLRING THE WX; WE ADVISED ATC THAT WE COULD RESUME ANY HDG THEY REQUIRED; THEY THEN ASSIGNED 240 DEGS AGAIN. AFTER A FEW MOMENTS; I DECIDED TO CANCEL IFR AND PROCEED TO MY DIRECT VFR. UPON CANCELLING IFR THE CTLR ASKED IF WE WERE CANCELING BECAUSE WE WERE ABOVE OUR ASSIGNED ALT OF 10000 FT. THE PNF REPLIED THAT WE HEARD THAT WE WERE ASSIGNED 12000 FT AND REPLIED THE SAME. THE CTLR THEN ASKED IF WE WOULD BE REQUESTING ADVISORIES AND GAVE US AN APPROPRIATE HANDOFF. AT THE TIME OF THE OCCURRENCE; THE FREQ WAS MODERATELY CONGESTED. OUR COMPANY HAS MANY WBOUND DEPS WITH ALTS OF 12000 FT. THE CTLR MAY HAVE THOUGHT WE WERE A PREVIOUS FLT TO A DIFFERENT DEST ALTHOUGH WE HEARD NO OTHER OUTBOUND COMPANY FLTS ON FREQ. THE CTLR WAS PERFORMING A COMMENDABLE JOB WITH MANY ACFT REQUESTING DEVS FOR WX IN THE TERMINAL AREA. I FEEL NOW THAT DUE TO MY FAMILIARITY WITH THE RTE AND THE DEP; I SHOULD HAVE QUESTIONED THE UNUSUAL ALT; AND NOT ASSUMED THAT IT WAS REQUIRED FOR TFC AVOIDANCE DUE TO OUR DEV. I HEARD 12000 FT AND VERIFIED IT WITH THE PNF AFTER HE READ BACK THE CLRNC AND DIALED IT INTO THE ALT ALERTER. I FEEL PERHAPS THAT ONLY INCREASED VIGILANCE ON OUR PART; WITH NO HESITATIONS FOR ASSUMPTIONS; IS CALLED FOR ON OUR PART WHEN THE ATC SECTOR IS SATURATED AND THE CTLR IS DOING HIS VERY BEST TO ACCOMMODATE MANY DEVS AND DEPS REQUESTED. FAMILIARITY WITH THE DEP AND RTE IN ADDITION TO THE CONFIDENCE IN THE CTLRS PROBABLY LED ME NOT TO QUESTION THE ALT AS NON- STANDARD DUE TO THE DEV AND THE WX CONDITIONS IN THE AREA.