POOR ATC PHRASEOLOGY FOR DEP CLRNC.

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WHILE IN SEQUENCE FOR TKOF ON RWY 18L AT DFW ARPT WE LISTENED TO THE TWR ISSUE THE CLRNC 'AT THE MARKER; FLY HDG 185.' WE DID NOT THINK MUCH OF THE CLRNC INITIALLY. WE ADJUSTED THE BRIGHTNESS ON OUR MARKER LIGHTS AND TURNED UP THE VOLUME. WE RECEIVED THE SAME CLRNC AND PRESSED ON OUR WAY. WHILE WAITING FOR OUR BRIGHTNESS ON OUR MARKER LIGHTS AND TURNED UP THE VOLUME. WE RECEIVED THE SAME CLRNC AND PRESSED ON OUR WAY. WHILE WAITING FOR OUR TURN FOR TKOF WE LOOKED AT THE SID TO SEE IF THE MARKER WAS DEPICTED AND IT WAS NOT. WE THOUGHT THERE PROBABLY OUGHT TO BE A DME CROSS REF/RADIAL REF TO BACKUP THE MARKER IF THEY ARE GOING TO ISSUE SUCH A CLRNC. THIS WOULD BE BENEFICIAL TO ACFT THAT WERE NOT MARKER EQUIPPED OR HAD INOP EQUIP. AS IT WAS; WE DID NOT GET A MARKER LIGHT INDICATION IN OUR ACFT BUT WE DID GET THE AURAL TONE. HAD WE FORGOTTEN TO TURN UP THE VOLUME IT COULD HAVE RESULTED IN A LATE TURN AND POSSIBLE CONFUSION. THE CONGESTED DFW FREQS WOULD MAKE IT DIFFICULT TO RESOLVE ANY CONFUSION THAT MAY HAVE RESULTED. OF COURSE; IF AN ACFT RECEIVED THIS CLRNC AND COULD NOT COMPLY (NO EQUIP/INOP EQUIP) THEY WOULD REQUEST AN ALTERNATE CLRNC. BUT ONCE AGAIN; WITH DFW'S CONGESTED FREQS THIS IS NOT A SIMPLE TASK. IF THIS IS TO BE A REGULARLY USED CLRNC; A GOOD SOLUTION WOULD BE TO DEPICT THE MARKER ON THE SID WITH A DME OR RADIAL CROSS REF TO CO-IDENT THE FIX POS. A LOT OF SIDS ALREADY ARE CLUTTERED WITH INFO BUT CONFUSION AT LOW ALT; ON DEP; AND WITH PARALLEL DEPS SHOULD BE AVOIDED IF AT ALL POSSIBLE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE DFW TWR PPS STATED THE MARKER TURN IS ISSUED FOR NOISE ABATEMENT. THE PHRASEOLOGY IS SUPPOSED TO BE 'AT THE MM FLY HDG.' PPS SAID HE WOULD PUT OUT A MEMO REMINDING CTLR'S TO USE THE WORD MIDDLE WHEN USING THIS PROC.