CLRED FOR THE 'BAY VISUAL' APCH; CREW THOUGHT THEY WERE CLRED FOR A VISUAL APCH AND ACTED ACCORDINGLY.

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DSNDING ON DOWNWIND FOR LNDG 16R SEATAC ARPT; FO TOLD APCH CTLR WE HAD ARPT IN SIGHT. CTLR SAID; 'CLRED BASE; TURN FINAL ABOVE 3200 MSL.' FO READ BACK 'CLRED VISUAL 16R; TURN FINAL ABOVE 3200 FT.' CTLR DID NOT MAKE ANY CORRECTIONS TO READBACK. AFTER TURNING BASE CTLR SAID; 'WE TURNED TOO EARLY AND SHOULD BE OVER TO BAY; FOR THE BAY VISUAL TO 16R.' WE TOLD HIM WE UNDERSTOOD 'TURN BASE AND TURN FINAL ABOVE 3200 FT.' THE APCH SUPVR ASKED ME TO CALL HIM. HE SAID HE LISTENED TO TAPE AND SAID THE CTLR CLRED US FOR 'BAY VISUAL TO 16R.' I DO NOT HAVE ACCESS TO OUR COCKPIT VOICE RECORDER TO HEAR WHAT WAS RECEIVED IN THE COCKPIT. HOWEVER; ALL 3 PLTS UNDERSTOOD 'CLRED BASE; TURN FINAL ABOVE 3200 FT;' AND THE FO READ BACK OUR UNDERSTANDING AND WAS NOT CORRECTED BY THE APCH CTLR. THE TITLE OF THE APCH 'BAY VISUAL' CAN BE INTERPRETED AS 'BASE' IF THE XMISSION IS BROKEN OR GARBLED. THE PROC NAME SHOULD BE CHANGED. NO TFC CONFLICT OCCURRED. THE PROC IS FOR NOISE ABATEMENT. THE PWR WAS AT IDLE UNTIL 2 1/2 TO 3 MI ON FINAL. SUPPLEMENTAL INFO FROM ACN 256656: CTLR SAID (I THOUGHT) 'TURN FINAL AT OR ABOVE 3200 FT; CLRED VISUAL APCH TO RWY 16R' AND THAT'S WHAT I READ BACK. CTLR DID NOT CORRECT ME.