ATR42 LANDED RWY 24 AT STL AND WAS CLRED TO LAND AND HOLD SHORT OF RWY 12R-30L. TWR ATCT AND GND CTLR WERE EXPECTING RPTR CAPT TO EXIT RWY 24 AT TXWY P; BUT RPTR EXITED TXWY BEFORE TXWY P WHICH WAS THE EXTENSION OF RWY 30R OR THE APCH END OF RWY 12L. THE AREA BTWN RWY 24 AND RWY 12L IS A CONCRETE PAD AND ADEQUATE FOR TAXIING; BUT BECAUSE RWY 30R WAS AN ACTIVE RWY; APPARENTLY AN ACFT TAXIING ACROSS THE EXTENDED END OF THE RWY WAS A PROB FOR THE CTLRS.

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TURNED OFF RWY 24 AT INTXN OF RWY 12L; TXWY C; AND RWY 24 DURING LAHSO OPS. ENTERED TXWY C INSTEAD OF TXWY P FIRST. AT NO TIME ENCROACHED ON RWY 30R/12L. INTXN NOT PROPERLY DEFINED ENOUGH FOR TWR TO SEE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR CAPT SAYS THAT THE ARPT PAGE 10-9 DOESN'T DEPICT VERY WELL THE AREA WHERE HE CLRED RWY 24; BUT A LARGE CONCRETE PAD AT THE NW END OF RWY 30R EXTENDS TO RWY 24. THAT PAD IS NOT CONSIDERED AS PART OF RWY 30R; THOUGH APPARENTLY THE TWR CTLR WAS UNHAPPY THAT RPTR CLRED THE RWY AT THAT POINT TO TXWY C BECAUSE RWY 30R WAS IN USE. CONSEQUENTLY; IF AN ACFT WERE TAKING OFF OR LNDG LONG; THERE WOULD BE AN ACFT TAXIING ACROSS THE END OF THE RWY. RPTR THINKS THAT SINCE THE PAD IS NOT PART OF THE RWY THERE SHOULD NOT BE A PROB WITH USING IT AS A TXWY TO CLR THE RWY; ESPECIALLY WHEN THEY WERE USING LAND AND HOLD SHORT PROCS. RPTR WAS CLRED TO LAND ON RWY 24 AND TO HOLD SHORT OF RWY 12R/30L AND WERE EXPECTING RPTR TO EXIT RWY 24 AT TXWY P; THOUGH THAT HAD NOT BEEN THE CLRNC.