ACR DSNDS BELOW ASSIGNED ALT NEAR SAX AFTER RECEIVING A PLT'S DISCRETION CLRNC.

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WHILE FLYING FROM BOS TO EWR; WE WERE INBOUND TO SAX; TRACKING ALONG V213. WE WERE INITIALLY LEVEL AT 12000 FT MSL. WE RECEIVED CLRNC TO CROSS 25 MI NE OF THE SAX VOR AT 7000 FT MSL. DURING OUR TRACK INBOUND; ZBW THEN CLRED US 'PLT'S DISCRETION' TO 8000 FT MSL. AS THE PF; I THOUGHT WE WERE CLRED AT OUR DISCRETION TO 8000 FT WITH THE ORIGINAL CLRNC TO CROSS 25 MI NE AT 7000 FT. I EITHER MISINTERPED THE CTLR'S CLRNC OR MISSED A XMISSION CANCELING THE CLRNC TO CROSS AT 7000 FT 25 ME NE OF SAX. AT APPROX 35 MI NE OF SAX; I STARTED TO DSND FROM 8000 FT SO AS TO CROSS 25 MI NE AT 7000 FT. AT ABOUT 32-33 MI OUT AT 7600 FT APPROX; WE RECEIVED A TCASII RA TO CLB; WHICH WE DID BACK TO 8000 FT. THE CTLR THEN ASKED ABOUT OUR ALT AND STATED WE WERE NOT CLRED TO 7000 FT MSL. I REALIZE THAT THE PARTICULAR SECTOR OF ZBW AIRSPACE WE WERE IN IS VERY BUSY AT THAT TIME OF DAY AND THE CTLR WAS WORKING A GREAT DEAL OF TFC. IT IS ALSO A BUSY TIME FOR OUR FLC; OBTAINING ATIS INFO; TALKING TO COMPANY FOR GATE INFO; ETA; ETC. I FEEL THAT A MORE EXPLICIT CANCELING OF THE 7000 FT XING RESTR WOULD HAVE AIDED IN ALLEVIATING THIS SIT.