CAPT OF AIR CARRIER LTT ACFT TAXIED ONTO TAXIWAY FROM RAMP WITHOUT CLRNC.

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MOST OF THE RAMP AREA AT PIT IS CTLED BY RAMP CTL. WHEN A COMMUTER LEAVES THE COMMUTER PARKING AREA; THE INITIAL CONTACT IS WITH RAMP CTL. THEY GIVE THE INITIAL TAXI CLRNC. I WAS TAXIING OUT FOR DEP. I LEFT BY WAY OF OSCAR; TAXIING TO TANGO. THE INSTRUCTIONS WE WERE GIVEN BY RAMP CTL WERE: 'RWY 28R; CONTACT GND WHEN ENTERING PAPA.' (NO 'HOLD SHORT OF PAPA' OR 'CONTACT GND AT PAPA' INSTRUCTION; WHICH IS THE USUAL INSTRUCTION). I INTERPRETED THE INSTRUCTIONS AS CLRED TO TAXI RWY 28R AND CONTACT GND CTL WHILE TRAVERSING PAPA. (NOTE: PAPA IS THE POINT WHERE RAMP CTL INTERFACES WITH GND. OSCAR TAXIWAY IS TOTALLY CTLED BY RAMP; AND TANGO AND PAPA ARE CTLED BY GND). GND CTL WAS UPSET BECAUSE THEY HAD ANOTHER COMMUTER ACFT ON PAPA GOING TO THE MAIN RAMP. THEY WANTED ME TO HOLD SHORT OF PAPA; LET THE OTHER GUY TAXI IN; AND THEN PROCEED ONTO TANGO. NOTE: I HAVE HAD SIMILAR EXPERIENCES WHEN I HAVE BEEN TAXIING IN. I AM ON PAPA AND SOMEONE CUTS IN FRONT (OSCAR TO TANGO). IT'S JUST A BUSY POINT ON THIS ARPT. IT'S THE MAIN POINT OVER WHICH ACFT TAXI IN AND OUT TO THE E HALF OF THE TERMINAL.