Illinois Additional Proof of Service is a document that is filed with the court to confirm that a defendant has been properly served with legal documents. It is also known as an Affidavit of Service. There are two types of Illinois Additional Proof of Service: Personal Service and Substitute Service. Personal Service is when a legal document is served to the defendant in person, usually by a process server. Substitute Service is when a legal document is served to someone else who is not the defendant, such as a family member, roommate, or a business partner, in order to provide the defendant with notice of the legal proceeding.