CL600 CREW WAS GIVEN A DEFACTO LAHSO CLRNC WHILE ON ROLLOUT AT DFW.

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ON ROLLOUT ON RWY 17C DFW; WE PLANNED AN M3 OR M5 HIGH-SPD EXIT. WE WERE A LITTLE FAST FOR M3 SO I TOLD FO (PF) TO LET IT ROLL TO NEXT HIGH-SPD M5. AS I WAS TURNING ONTO M5 (STARTING TO TURN); ABOUT 25-35 FT OFF CTRLINE OF RWY 17C THE TWR CTLR SAID; 'ACR X (OUR CALL SIGN) TAKE NEXT HIGH SPD.' I MISSED NEXT PART AND I HEARD 'BRAVO HOLD SHORT RWY 17R.' THE RAPID SPEAKING AND HIGH WORKLOAD AT THAT MOMENT MADE IT HARD TO UNDERSTAND. SINCE I WAS MOVING TOWARDS M5 AT THE TIME; AND COMMITTING TO IT WHEN CTLR SAID TAKE NEXT HIGH-SPD; I COULD ONLY ASSUME HE MEANT M6. SO; I CONTINUED TO ROLL ON RWY AND CAUSED ANOTHER ACFT TO GAR. TOO MANY INSTRUCTIONS AND SPOKEN TOO QUICKLY AT A HIGH WORKLOAD TIME. ALSO; ATL TWR WILL TELL YOU PARTICULAR HIGH-SPD IF THEY WANT YOU ON THE ONE YOU ARE APCHING. IN THIS CASE; DFW CTLR SAID (NEXT) HIGH-SPD (TO ME; M6); SO THAT'S WHAT I DID. THIS HAS HAPPENED TO ME BEFORE WHEN I WAS AN FO. THE CAPT I WAS WITH DID THE SAME THING; SAME INSTRUCTION; SAME RWY. WAS M5 BLOCKED BY ANOTHER ACFT? WAS IT CLOSED? IT WAS DARK. WHEN HE SAID NEXT HIGH-SPD TO ME; IT WAS M6.