Often it's helpful to provide information in an email to an applicant or admins that's been collected or calculated during a process.
When setting up a workflow you may use the "@" sign to call up a list of all fields and formulas available to include in an email, step or section instructions, or in the Completion message for the workflow.
To include the "File Number" field in a completion message you may type the "@" sign, then in this case begin typing "file" to bring up the correct field.
Simply hit "enter" and the field data will be included. You can be sure it's working if the field or formula name appears on a darker background.