AN MD80 CREW DEPARTING ON THE IND ROCKY5 QUESTION THE AMBIGUOUS WORDING ABOUT THE INITIAL DEP HDG OF 227 OR 212 DEGS.

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CLRED VIA PDCS PRINTED CLRNC FOR THE ROCKY5 DEP. VERBIAGE IN SID SAYS; 'CLB RWY HDG 227 DEGS UNTIL IVZK D2.6...THEN FLY HDG ASSIGNED 227 OR 212 DEGS. EXPECT RADAR VECTORS.' WE BRIEFED THE SID AND FROM OUR READING; 'FLY ASSIGNED HDG' MEANT THAT HDG IN THE SID WAS ASSIGNED. APPARENTLY; THAT REALLY MEANS; 'FLY HDG THAT WILL BE ASSIGNED BY TWR.' BUT IT DOESN'T SAY THAT; EVEN THOUGH IT CAN (AND DID) MEAN THAT TO SOMEONE READING IT PRIOR TO TKOF. WHEN THE TWR CTLR CLRED US FOR TKOF AND SAID; 'FLY RWY HDG;' THAT SOUNDED LIKE CONFIRMATION OF THE SID AS WE BRIEFED IT; WHICH STARTS OUT WITH; 'FLY RWY HDG.' HAD HE SAID; 'FLY HDG 227;' HE WOULD HAVE FULFILLED THE SID 'ASSIGNED HDG' STIPULATION REGARDING HDGS AFTER TKOF. IT DOESN'T SAY; 'FLY RWY HDG OR ASSIGNED HDG 212 OR 227.' SO WE ASSUMED WE WERE ON THE SID SINCE HE HADN'T SPECIFIED ANYTHING OTHER THAN WHAT'S ON THE FIRST LINE OF THE SID. I CAN'T RECALL MANY SID'S WHERE YOU ARE TO FLY AN EITHER/OR HDG WITHOUT SOMEWHERE INCLUDING AN INDICATION THAT THE HDG WILL BE ASSIGNED BY TWR. IN THIS CASE; WHY THE TWO HDGS IN THE SID IF IT'S A TWR ASSIGNMENT IN ALL CASES? WHY THE VERBIAGE; 'FLY ASSIGNED HDG 212 OR 227;' WHICH IS CONFUSING WITHOUT THE CRUCIAL INFO; AS ASSIGNED BY TWR? SPEAKING WITH THE TRACON SUPVR LATER; HE SAID THIS AMBIGUITY IS A CONSTANT SOURCE OF CONFUSION WITH FLTS OUT OF THERE. HE SAID PLT ALWAYS ASK; 'DO YOU WANT US TO FLY THE SID OR YOUR HDG?' I CAN SEE WHY THIS IS CONSTANT SOURCE OF CONFUSION; BECAUSE WITHOUT AN INDICATION THAT; 'TWR WILL ASSIGN A HDG;' IN WHICH CASE THE 212 OR 227 DEG HDG DOESN'T NEED TO BE IN THE SID INSTRUCTIONS -- THERE ARE SEVERAL WAYS TO READ THIS PROC; NOT ALL OF WHICH CONFORM TO THE TWR'S INTENT.