Since you must usually give a reason for using a private vehicle for a business trip in the German Public Sector, you can enter a reason in the travel expense manager (PR05) by choosing Reason.
You can also fill the free text fields for entering the reason with certain standard texts. You make the settings for this in the Customizing for Travel Expenses under Dialog and Travel Expenses Control → Dialog Control → Assign Standard Text for Private Vehicle Use (Pub.Sector)