AIR AMBULANCE LR36 CAUSES PRECAUTIONARY GAR AT BOS RESULTING IN RWY INCURSION.

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WE WERE AT BOSTON LOGAN INTL. I WAS THE 'PNF.' I RECEIVED OUR TAXI CLRNC; WHICH WAS 'TAXI TO TXWY B HOLD AREA VIA TXWY K.' AS WE BEGAN TO TAXI; I WAS LOOKING DOWN AT THE CHART AND GUIDING THE CAPT OUT OF RAMP PARKING. THE TXWY B AREA IS VERY CONFUSING AND I KEYED IN ON THE TXWY B THINKING WE WOULD COME TO A HOLD SHORT LINE PRIOR TO RWY 9. AS WE CONTINUED WE BOTH AGREED THAT SOMETHING DID NOT SEEM RIGHT SO WE STOPPED. JUST THEN THE CTLR CAME ON THE RADIO AND ADVISED US TO STOP IMMEDIATELY (WHICH WE HAD ALREADY DONE). RWY 9 HAS A STOPWAY ON IT. WE TAXIED PAST THE TXWY B HOLDING AREA APPROX 1 PLANE'S LENGTH. WE WERE RIGHT UP TO THE STOPWAY FOR RWY 9. A COMMUTER WAS ON APPROX 1 MI FINAL AND 800 FT. HE ELECTED TO 'GAR.' WE WERE THEN GIVEN NEW TAXI INSTRUCTIONS AND TOLD TO CALL THE TWR WHEN WE GOT TO OUR DEST. WE CALLED AND SPOKE WITH A SUPVR WHO TOLD US NO ACTION WOULD BE TAKEN AND THAT THEY KNOW HOW CONFUSING THAT TXWY B HOLD AREA IS AND THIS HAS HAPPENED FREQUENT IN THE PAST. I THINK WHAT CAUSED THE CONFUSION WAS THAT I THOUGHT WE WOULD COME UP TO A HOLD SHORT LINE OR AN OPEN AREA. HOWEVER THE TXWY B HOLD AREA IS A SPOT-LINE R ON TXWY B. THE TXWY ALSO MAKES ALMOST A 90 DEG TURN WHICH IS WHERE THE TXWY B HOLD IS AND CONTINUES UP TO RWY 9. I SHOULD HAVE HAD THE CAPT TAKE A LOOK AT THE CHART OR RECEIVED 'PROGRESSIVE TAXI INSTRUCTIONS.' I THOUGHT I HAD IT FIGURED OUT. IT WOULD HAVE HELPED TO MAYBE HAVE A 'BLOW-UP' OF THAT AREA OR MAYBE SOME SORT OF SIGN. WHENEVER I GO INTO A NEW OR STRANGE ARPT I WILL ALWAYS ASK FOR 'PROGRESSIVE INSTRUCTIONS.'