ACR MLG CLEARED TO RWY 21 CROSSED HOLD LINE RESULTING IN TWO GO AROUNDS CAUSED BY APPARENT TRANSGRESSION.

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WE WERE CLRED TO TAXI RWY 21. WE UNDERSTOOD (CAPT AND MYSELF) THIS TO MEAN TAXI TO THE DEP END OF #21 SINCE NO 'HOLD SHORT' INSTRUCTIONS WERE GIVEN. SEVERAL SMALL ACFT WERE DEPARTING FROM AN INTERSECTION AHEAD OF OUR CRAFT SO WE WAITED ON THE PARALLEL TAXIWAY UNTIL OUR TURN TO MOVE ALONG. SINCE WE DIDN'T WANT AN INTERSECTION DEP WE CONTINUED ALONG THE PARALLEL AS CLRED; SO WE THOUGHT. TWR ADVISED TWO LANDING ACFT TO GO-AROUND BECAUSE WE HAD VIOLATED THE 'ARPT SAFETY ZONE.' WE ALSO RECEIVED A STERN LECTURE THAT WE WERE TO HOLD AT THE PAINTED 'HOLD' LINE AND NOT TO TAXI TO THE END UNTIL CLRED BY TWR. THERE IS IN FACT A 'HOLD' LINE PAINTED ON THE PARALLEL TAXIWAY. WE UNDERSTOOD THIS LINE TO REPRESENT THAT THRESHOLD FOR AN INTERSECTION DEP. AIRMEN INFORMATION MANUAL IS SPECIFIC IN THAT THIS LINE SEPARATES AN ACTIVE TAXIWAY FROM AN ACTIVE RWY; NOT DIVIDING A TAXIWAY. ALSO WE COULD FIND NO REFERENCE ANYWHERE ON OUR CHARTS; IN THE FAR'S; OR IN THE AIM TO THE 'ARPT SAFETY ZONE'. I HAD BEEN THERE BEFORE AND I DID KNOW OF THE 'HOLD LINE'. HOWEVER; PREVIOUSLY I HAD ALWAYS BEEN TOLD TO 'HOLD SHORT AT THE THE HOLD LINE' AND WE DID. ON THIS PARTICULAR INSTANCE WE WERE CLRED TO 'TAXI TO RWY 21'; A DIFFERENT CLRNC ALTOGETHER. I RECOMMEND THIS ARPT SAFETY ZONE BE DEPICTED ON OUR CHARTS WITH A CAUTION NOTATION TO DRAW ONES ATTN TO AN UNUSUAL SITUATION. ALSO ATC NEEDS TO ADVISE CTLRS TO STATE CLEARLY WHAT THEY WANT US TO DO; NOT ASSUME WE KNOW WHAT THEY ARE THINKING.