A300 CREW TAXIED AROUND TXWY U AS INSTRUCTED BY THE ATCT GND CTLR. THE ACR FLT PLAN NOTES AND THE FOM NOTES RESTR ACFT TAXIING ON TXWY U. THE CAPT FOUND THE NOTES CONFUSING AND AMBIGUOUS.

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AFTER LNDG ON RWY 8 AT MIA; WE CLRED THE ACTIVE TO THE S AND JOINED PARALLEL TXWY L BTWN RWY 8 AND RWY 9L. GND TOLD US TO TAXI E WHILE DETERMINING IF OUR GATE WAS OPEN. WE TOLD THEM WE WOULD HAVE TO HOLD FOR ABOUT 20 MINS AND THEY INSTRUCTED US TO CONTINUE E AND TO HOLD SHORT OF TXWY L11. WE DID; AND I SHUT DOWN THE #1 ENG WHILE WAITING. DURING OUR WAIT; I ONCE AGAIN REVIEWED THE INFO ON THE FLT PLAN CONCERNING USE OF THE 'U' ON THE EASTERN END OF RWY 9L. LATER; GND CALLED AND ADVISED US TO CONTINUE E AND GO AROUND THE 'U' AND HOLD SHORT OF TXWY M11 ON THE OTHER SIDE OF RWY 9L. SIMULTANEOUSLY; RAMP TOLD US OUR GATE WAS OPENING UP AND WE SHOULD HEAD TOWARD THE GATE. I HESITATED WHILE STARTING ENG #1 AND THINKING ABOUT THE WHOLE SIT. GND THEN CALLED US BACK AND STATED THAT I WAS NOT TO TURN AND CROSS AT TXWY L11; BUT TO CONTINUE STRAIGHT AHEAD. AS WE CONTINUED E; THE FO ASKED IF THIS WAS LEGAL AND I STATED THAT I UNDERSTOOD IT TO BE LEGAL AS LONG AS THERE WAS NOT AN ACFT TAKING OFF OR LNDG WHILE WE WERE IN THE 'U.' I ALSO STATED THAT B757'S AND SMALLER WERE ALLOWED TO USE IT WITHOUT RESTR. TO ME; THIS IMPLIED THAT LARGER ACFT COULD USE THE 'U' WITH RESTRS --WHICH I TOOK TO BE ACFT TAKING OFF OR LNDG. AS WE APCHED THE 'U;' I ASKED THE FO TO VERIFY THAT NO ACFT WERE TAKING OFF. HE STATED THERE WAS ONE IN POS; BUT IT WAS NOT TAKING OFF. WE EXPEDITED AROUND THE 'U' WHILE COMMENTING ON THE FACT THAT THIS DID NOT FEEL COMFORTABLE REGARDLESS OF WHAT SIZE ACFT WE WERE IN. WE ALSO NOTED THAT AN ACFT DID TAKE OFF OVER A B757 IN THE 'U.' ONCE STOPPED AT TXWY M11; I NOTICED A B767 WAITING AT TXWY L11 TO CROSS RWY 9L; SO I PULLED OUT THE PAPERWORK ONCE AGAIN TO READ THE INFO ON THIS TYPE OF SIT AND STILL WAS NOT SURE HOW IT WAS SUPPOSED TO WORK. SINCE IT DID NOT SAY I WAS NOT ALLOWED TO USE THE 'U;' I FELT I HAD COMPLIED WITH THE IMPLIED RESTR. WE EVENTUALLY CONTINUED TO THE GATE. WHILE ON THE WAY HOME; I CONTACTED THE CHIEF PLT ABOUT ANOTHER ISSUE AND BROUGHT THIS UP TO CLARIFY IT IN MY MIND. THAT IS WHEN I FOUND OUT I WOULD NEED TO FILE THIS RPT. IT SHOULD BE NOTED THAT WHEN RESTRS SUCH AS THIS ARE PLACED ON CERTAIN ACFT; IT IS USUALLY VERY SPECIFIC ABOUT WHICH ACFT CAN AND CANNOT USE A GATE; TXWY; ETC. I DO NOT FEEL THE MESSAGE ON THE FLT PLAN WAS CLR. NONETHELESS; AS CAPT; I DID MAKE A MISTAKE AND ACCEPT THAT AS MY RESPONSIBILITY AND AS A LEARNING EXPERIENCE.