FLC OF AN LGT DSNDED FROM ASSIGNED EARLY DUE TO A MISUNDERSTANDING OF THE CLRNC.

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FLT WAS ON A DELAYING VECTOR (HDG 180 DEGS S FROM OLM VORTAC). FO WAS PF WHILE I (CAPT) WAS ON #2 RADIO TALKING TO OUR COMPANY DISPATCHER. FO WAS ISSUED A CLRNC TO PROCEED DIRECT OLM AND RESUME THE OLYMPIA 2 ARR TO SEA. ACFT WAS LEVEL AT 17000 FT MSL; THE LAST ASSIGNED ALT. FO MISUNDERSTOOD CLRNC AND THOUGHT HE HAD BEEN CLRED TO 'DSND VIA THE OLYMPIA 2 ARR.' HE BEGAN A DSCNT TO 16000 FT. WE WERE NOT YET BACK TO OLM AND WERE NOT ON A SEGMENT OF THE OLYMPIA 2 ARR. PASSING APPROX 16300 FT DSNDING; THE CTLR INFORMED HIM THAT WE HAD NOT BEEN CLRED TO DSND FROM 17000 FT. THE FO IMMEDIATELY INITIATED A CLB BACK TO 17000 FT. THE CTLR THEN ISSUED A CLRNC TO DSND TO 16000 FT AND; AFTER REACHING OLM; TO 'DSND VIA THE OLYMPIA 2 ARR.' I HAD JUST FINISHED TALKING TO OUR DISPATCHER AND CAME BACK ON THE ATC RADIO WHEN THIS LAST CLRNC WAS ISSUED. IN MY FO'S DEFENSE I WOULD LIKE TO SAY THAT I HAVE BEEN WITH MANY OTHER PLTS WHO FIND THESE 'DSND VIA' CLRNCS CONFUSING (ESPECIALLY THE ONES IN THE LOS ANGELES AREA). IT WOULD BE VERY HELPFUL IF THE CTLR ALWAYS ISSUED A 'MAINTAIN ALT' CLRNC AT THE TIME AN ARR CLRNC IS ISSUED UNLESS THE 'DSND VIA' CLRNC IS BEING ISSUED AT THE SAME TIME. IN THE CASE OF OUR CLRNC; IT WOULD HAVE ELIMINATED THE CONFUSION IF THE CTLR HAD SAID 'CLRED DIRECT OLYMPIA. AFTER OLYMPIA CLRED VIA THE OLYMPIA 2 ARR; MAINTAIN 17000 FT.' I WOULD SAY A CONTRIBUTING FACTOR WAS THAT I WAS 'OUT OF THE LOOP' AND DID NOT HAVE THE VOLUME UP ON THE ATC RADIO WHILE I TALKED TO DISPATCH.