A CL65 FLT CREW QUESTIONS THE TAXI CLRNC GIVEN BY LCL CTLR AFTER LNDG AT BOS; MA.

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AFTER LNDG ON BOS RWY 4R; TWR INSTRUCTED US TO EXIT L ONTO RWY 33R. I'M NOT CERTAIN; BUT I BELIEVE THAT THE CTLR THEN INSTRUCTED US TO TAXI TO THE RAMP; BUT HOLD SHORT OF RWY 15R (THE FO THOUGHT HE SAID 'HOLD SHORT OF RWY 4R'; EVEN THOUGH WE HAD JUST LANDED OF RWY 4R). THE FO AND I BOTH HAD OUR ARPT DIAGRAMS OUT AND WERE DISCUSSING 'WHAT DID THE CTLR SAY?' I HAD BEEN TAXIING VERY SLOWLY; AND WAS ALMOST STOPPED SO THAT WE COULD GET A CLARIFIED CLRNC WHEN THE CTLR CALLED US AND TOLD US TO 'HOLD SHORT OF RWY 4L.' WE WERE ALREADY NEARLY STOPPED; BUT DID STOP SHORT OF RWY 4L. THE CTLR THEN INSTRUCTED A LNDG ACFT TO GO AROUND. I'M NOT SURE WHY; WE WERE NOT INTRUDING ONTO HIS LNDG RWY. MY ASSUMPTION IS THAT IT WAS A PRECAUTIONARY MOVE ON THE PART OF THE CTLR. WE MAY ALL HAVE BEEN UNCERTAIN. BOSTON CAN BE A VERY CONFUSING ARPT CONFIGN TO TAXIING CREWS. THERE ARE ALREADY 5 RWY INCURSION HOT SPOTS LISTED ON THE ARPT DIAGRAM. THIS WAS AN INTERESTING SIT; BECAUSE WHEN TAXIING ON A RWY; THERE ARE NO 'HOLD SHORT' LINES FOR INTERSECTING RWYS. MY FO AND I WERE VERY TIRED AT THE END OF A ROUGH FLYING DAY; BUT WE WERE BOTH VERY CAUTIOUS ABOUT THE RWY LAYOUT AND OUR POS. I WAS STOPPING BECAUSE WE JUST WERE NOT SURE ABOUT OUR TAXI CLRNC. THERE ARE TIMES WHEN IT IS BETTER TO 'STOP FIRST; AND ASK QUESTIONS LATER.'