FLC OF ACR MLG ACFT INADVERTENTLY DEVIATED FROM THE SID DEP ASSIGNED.

Narrative

TKOF FROM RWY 16L WAS NORMAL; CLBOUT COMMENCED ON RWY HDG. WE WERE ANTICIPATING RADAR VECTORS TO INTERCEPT SEA J70 ELMAA; ETC; WHICH WAS THE RTE LISTED ON THE PDC MESSAGE XCHKED AGAINST OUR FILED FLT PLAN. SEATTLE DEP QUESTIONED US IF WE WERE IN THE TURN. WE REPLIED NEGATIVE; WE WERE RWY HDG EXPECTING VECTORS. THE PORTION OF THE PDC MESSAGE CONTAINING THE RTW WAS AGAIN REFED; AND IT REFLECTED OUR FILED RTE OF SEA J70 ELMAA; WITH NO MENTIONED OF ANY SID. DEP THEN QUESTIONED WHICH SID WE WERE ASSIGNED. AGAIN REFING THE PDC MESSAGE; WE NOTICED THAT THE ELMAA 3 DEP WAS LISTED; HOWEVER; IT WAS PRINTED ON THE TOP PORTION OF THE PDC MESSAGE; NOT NEAR THE RTE LINES WHICH ARE ALWAYS ON THE BOTTOM PORTION OF THE MESSAGE. OTHER ARPTS PLACE THE ASSIGNED SID ON THE BOTTOM OF THE PDC MESSAGE AT THE START OF THE ASSIGNED ROUTING. THE PLACEMENT OF THE SID IN THE PORTION OF THE PDC MESSAGE RESERVED FOR REMARKS CAUSED BOTH PLTS TO BELIEVE NO SID WAS ASSIGNED; SINCE NORMAL HABIT PATTERNS FOR BOTH PLTS XCHKING OF FILED ROUTING VERSUS ASSIGNED ROUTING DID NOT ENCOMPASS REF TO THE REMARKS PORTION OF THE PDC MESSAGE.