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One way is to use a workflow to step through the Item lines and only update the Department (main) from the first line. 

I'm not sure how to add that as a condition. I have a Set Field Value action for the header field and I don't see any option to copy the value from the first line.

I'm suggesting do it in a Sublist Action Group (Items), the workflow Action, Set Field Value then allows you to choose if you are setting Department (Main) or Department (Line), you'll be setting Main.

in the Workflow Condition you can set Line = 1 and use that to be the action when you set the Department Main

Oh I can set the main even if using Sublist Action Group? Thank you.

Make sure you are using the Sublist group 