Towards the end of the selection process, potential employers will typically ask for your references. Using the reference letter is your best way to provide them with this information.
You shouldn’t include this document when you submit your resume, unless the advertisement specifically asks you to.
The reference letter is a list of the contact information for your references, and your connection to them. You may have more than one email, or phone number for your references, it’s always worth checking with them which to use.