ACR FLT CREW MISUNDERSTANDS CLRNC TO RESUME THE CHATY 2 DEP FROM LAX AND TURNS BACK TO INITIAL HDG OF 040 DEGS.

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TKOF AND DEP OPS TO THE E ARE RARE OUT OF LAX LESS FAMILIAR TO BOTH PLTS AND CTLRS. THE DEP PROC CALLS FOR A SPECIFIC HDG UNTIL RECEIVING 'VECTORS TO V-23.' ATC NEVER MENTIONED THAT OUR VECTORS WERE FOR THAT PURPOSE. OUR UNUSUAL AND IMMEDIATE VECTOR WAS SAID TO BE FOR TFC. WHEN HE SIMPLY SAID TO 'RESUME' THE DEP WITH NO FURTHER INSTRUCTIONS; WE WERE LEFT WONDERING WHETHER WE SHOULD RETURN TO THE SPECIFIED HDG ON THE CHART AND AWAIT VECTORS ON COURSE (WHICH IS CUSTOMARY ON THE GORMAN 4 DEP WE NORMALLY FLY); OR IF HE HAD SOMETHING ELSE IN MIND. HAD WE BEEN GIVEN VECTORS TO 'INTERCEPT' AN AIRWAY OR COURSE; OR CLRNC DIRECTLY TO A FIX; IT WOULD HAVE BEEN AN UNEVENTFUL DEP. RADIO CONGESTION PREVENTING VERIFICATION OR CLARIFICATION. THE FMS DID NOT SEQUENCE WHEN USING THE HDG BUG AS A REF AS IT WOULD HAVE IF WE WERE FLYING IN FMS HDG MODE; THUS THE DEP RADIAL DID NOT SHOW ON THE MFD AND SITUATIONAL AWARENESS WAS DEGRADED (SMALL IMPACT). CONTINUOUS LIGHT TO MODERATE CHOP; LOW CEILINGS; LOW/SLOW OPS AND TIGHT MANEUVERING FOR TFC ADDED TO AN ALREADY BUSY PART OF THE FLT.