ACR MLG UNAUTH RWY ENTRY CAUSES GAR FOR ACFT COMPLETING AN ILS APCH IMC.

Narrative

INITIAL TAXI CLRNC WAS FROM GATE TO RWY 23L VIA THE RAMP AND TAXIWAY 'S'. THERE WERE SEVERAL ACFT STOPPED ON THE RAMP AND MUCH CHATTER ON GND FREQ SO WE STOPPED ON THE RAMP TO VERIFY THE TAXI CLRNC. WE EXPECTED TO HEAR RAMP; SIERRA; LIMA; 23L. JUST AS WE WERE ASKING FOR CLARIFICATION FROM GND WE WERE TOLD TO HOLD OUR POS AND CONTACT TWR FOR FURTHER CLRNC. THERE WAS A LOT OF CHATTER ON TWR FREQ AS THEY WERE ASKING MOST ACFT IF THEY COULD ACCEPT THE SHORTER RWY 28 FOR DEP. WE VOLUNTEERED THAT WE COULD ACCEPT RWY 28 BUT DID NOT RECEIVE A RESPONSE FROM TWR. WE WERE THEN TOLD TO CROSS RWY 23L AT SIERRA; BACK TAXI DOWN 23R AND HOLD SHORT OF RWY 28. WHILE HOLDING SHORT OF RWY 28; ON RWY 23R (HDG 050) 1 ACFT DEPARTED ON RWY 28 AND 1 LANDED ON 23L. WE WERE THEN TOLD TO TAXI INTO POS AND HOLD RWY 23L. WHILE TAXIING INTO POS ON 23L WE HEARD TWR TELL AN ACFT TO GAR (NOT VISIBLE TO US DUE TO WX) AND TWR TOLD US THAT WE HAD BEEN CLRED ON TO RWY 23R POS AND HOLD. WE HAD INITIALLY BEEN TOLD TO TAXI TO RWY 23L. WE APPARENTLY DID NOT RECOGNIZE THE CHANGE TO RWY 23R IN THE CLRNC FROM TWR FOR INTO POS AND HOLD. TWR OR GND SHOULD HAVE TOLD US OF A CHANGE IN INTENDED RWY. ACKNOWLEDGEMENT OF CLRNC SHOULD HAVE INCLUDED WHICH RWY. (ROGER; CLRED ONTO RWY 23L; POS AND HOLD). CORRECT TERMINOLOGY FROM TWR: CONTINUE BACK TAXI DOWN 23R; CROSS RWY 28 INTO POS AND HOLD RWY 23R; WOULD HAVE PREVENTED CONFUSION.