B737-300 FLT CREW INADVERTENTLY ENTERS RAMP 11 AT SDF WHEN CLEARED TO RWY 35L VIA CHARLEY.

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AFTER PUSHBACK I CALLED GND CTL FOR TAXI. GND TOLD US TO TAXI TO RWY 35L. WE STARTED TO TAXI WHEN GROUND CTL TOLD US TO MAKE NEXT L TURN AND USE TXWY CHARLEY. AN ACFT HAD JUST LANDED AND LOOKED LIKE IT WOULD TURN OFF AT TXWY B6 AND WOULD BE TAXIING NORTH ON TXWY B. WE CONTINUED TO RWY 35L ON TXWY C. WE WERE NUMBER 1 FOR DEP SO I SWITCHED TO TWR FREQUENCY AROUND THE TXWY B3 INTXN AND COMPLETED MY BEFORE TAKEOFF FLOW. TWR ASKED IF WE WERE WITH HIM. I RESPONDED YES AND WE WERE READY TO GO. HE TOLD US TO GO BACK TO GND THEY WANTED TO TALK TO US. WHEN I SWITCHED BACK TO GND I HEARD THE CTLR SAY; 'ACR X HOLD YOUR POSITION.' WE STOPPED. THE CTLR THEN SAID WE WERE ON ANOTHER CARRIER'S RAMP. I ASKED WHAT FREQUENCY WE NEED TO TALK TO THEM ON. HE RESPONDED 'STANDBY; I'M TRYING TO COORDINATE WITH THEM.' A MINUTE OR TWO LATER THE CTLR TOLD US TO MAKE A L 180 DEG; TAXI BACK TO TXWY B3 AND JOIN TXWY BRAVO. I APOLOGIZED AND SAID THAT WE THOUGHT THAT WE WERE SUPPOSE TO FOLLOW TXWY CHARLEY ALL THE WAY TO THE RWY. DEP WAS NORMAL. ON THE ARPT PAGE IT APPEARS THAT TXWY CHARLEY CONTINUES PARALLEL TO TXWY BRAVO ALL THE WAY TO THE STANDARD DEP POINT ON RWY 35L. THERE IS A LABEL OF RAMP 11 DOWN BY THE END OF THE RWY BUT NO MENTION OF A FREQUENCY OR RESTRS. THERE WAS 1 OF THE OTHER CARRIER'S ACFT PARKED OFF TO THE L OF WHAT WE THOUGHT WAS TXWY C BUT NOTHING TO OBSTRUCT US FROM TAXIING TO THE RWY. THE CAPT SAID HE DID SEE A SMALL SIGN WITH SOMETHING ABOUT A RAMP ON IT AFTER WE MADE OUR 180 DEG TURN AND WERE LEAVING. I COULD NOT SEE THE SIGN FROM MY SIDE OF THE ACFT. I DON'T KNOW IF I SWITCHED FREQUENCY JUST AS THE GND CTLR WAS TELLING US TO JOIN TXWY BRAVO OR IF HE JUST FORGOT ABOUT US UNTIL IT WAS TOO LATE. THERE COULD BE BETTER MARKINGS ON THE ARPT OR MAYBE A FREQUENCY ON THE ARPT PAGE THAT WOULD BETTER ALERT THE CREW THAT THIS WAS A CONTROLLED RAMP.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR SPECIFIED THAT THE REVISED TAXI CLRNC WAS TO TURN L ON TXWY CHARLEY TO RWY 35L ACCOUNT INBOUND TFC ON TXWY BRAVO. NO MENTION OF A TRANSITION BACK TO BRAVO AT AN INTERSECTING TXWY WAS MADE. RPTR HAD NOT RETURNED TO SDF AND WAS UNABLE TO PROVIDE ANY MORE SPECIFIC DETAIL REGARDING THE SMALL SIGN MENTIONED BY THE CAPT AFTER THEIR 180 DEG TO EXIT RAMP 11. HE REITERATED HE THOUGHT IT MAY HAVE SAID SOMETHING ABOUT THE APCHING RAMP AREA BUT THE FLT CREW WAS UNABLE TO READ IT WHILE TAXIING NORTHBOUND. RPTR ALSO MENTIONED MINOR INITIAL CONFUSION REGARDING THE IDENTITY OF TXWY B DUE TO THE POOR LABELING ON THE COMMERCIAL PRODUCED ARPT DIAGRAM USED BY THE ACR.