CAPT OF AN LGT STARTED DSCNT EARLY DUE TO MISINTERP OF HIS CLRNC RESULTING IN ATC INTERVENTION AND CLRNC TO A LOWER ALT SO THAT HE DID NOT HAVE TO CLB BACK TO ASSIGNED.

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INBOUND TO SFO; ASSIGNED 11000 FT; ON THE FMS BRIDGE VISUAL RWY 28R. WE WERE CLRED 'DIRECT ARCHI TO INTERCEPT THE FMS BRIDGE VISUAL RWY 28R.' I WAS REVIEWING THE ARR AGAIN AND WHEN I LOOKED UP; WE WERE PASSING THROUGH 10000 FT. I ASKED THE CAPT IF WE HAD BEEN CLRED OUT OF 11000 FT AND HE SAID HE THOUGHT WE WERE CLRED FOR THE APCH. I TOOK THE AIRPLANE; WHICH WAS ON AUTOPLT; AND STARTED A CLB TOWARD 11000 FT. AT THAT TIME ATC CALLED AND ASKED OUR ASSIGNED ALT. THE CAPT RESPONDED THAT WE WERE ON THE ARR PROFILE. ATC THEN CLRED US TO MAINTAIN 8000 FT. NO OTHER DISCUSSION FOLLOWED AND WE WERE SUBSEQUENTLY CLRED THE ARR PROFILE. I BELIEVE THE PROB WAS THAT THE CAPT THOUGHT WE WERE CLRED FOR THE FMS APCH; WHEN IN REALITY WERE JUST SUPPOSED TO TRACK INBOUND ON IT AT 11000 FT. I DIDN'T NOTICE THE CAPT START THE DSCNT. EARLIER THE CAPT HAD ME SET 7000 FT IN THE ALT WINDOW -- THE NEXT ALT REQUIRED AT ARCHI. OUR SOP SAYS THAT WE ONLY SET A CLRED ALT INTO THE WINDOW. THAT SHOULD HAVE CLUED ME TO THE FACT THE CAPT THOUGHT WE WERE CLRED THE APCH; BUT I MISSED IT. I THINK VERBIAGE HAS A LOT TO DO WITH WHAT OCCURRED. THE WORDS 'CLRED TO INTERCEPT THE APCH' AND 'CLRED THE APCH' ARE VERY SIMILAR. I UNDERSTOOD WHAT ATC WANTED US TO DO; BUT THE CAPT MISINTERPED. I WAS NOT WATCHING WHEN THE DSCNT WAS INITIATED OR I COULD HAVE PREVENTED IT. IF ATC HAD USED THE TERM 'CLRED TO INTERCEPT; MAINTAIN 11000 FT;' THINGS WOULD HAVE BEEN DIFFERENT.