Internal Revenue Code section 6104 states that if an organization described in section 501(c) or (d) is exempt from taxation under section 50(a) for any taxable year, the application for exemption is open for public inspection. This includes all supporting documents, any letter or other documents issued by the IRS concerning the application, and certain annual returns of the organization. Form 4506-A is used to request public inspection or a copy of these documents.
The latest form for Form 4506-A and Form 4506-B -- Request for a Copy of Exempt or Political Organization IRS Form, Application, or Letter expires 2022-06-30 and can be found here.