Reason Code on Sales Order

It appears that the reason code on a sales order is not committed to the db unless the reason code is entered into the header and then into the detail.  If entering the reason code on just the detail line, it does not appear to be committed to be the db when the order is saved.  Is there a reason for this behavior?  I could not find, or did not look hard enough, for documentation explaining how this works.  If anyone has some insight, please pass along.  Thanks.

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I understand that what happens is when you populate it at the header the bsfn copies it to the detail lines.