Illinois Proof of Service of Notice to Withhold Income is a form that is used to document that legal documents related to withholding income have been served to the appropriate parties. This type of form is used to demonstrate that a Notice to Withhold Income has been served to the employee, employer, and Illinois Department of Revenue. The form must be signed by a court clerk, sheriff, or other authorized agent to be considered legally binding. There are two types of Illinois Proof of Service of Notice to Withhold Income: in-person service and substituted service. In-person service requires the authorized agent to hand-deliver the documents to the employee or employer. Substituted service requires the authorized agent to deliver the documents to a person of suitable age and discretion at the place of business of the employee or employer.