Final answer:
A business receives an EIN letter from the IRS as verification of their employer identification number, which serves as proof of the business's name, address, and EIN for various official purposes.
Step-by-step explanation:
The document a business receives as verification of their employer identification number (EIN) from the IRS is known as an EIN letter. This document serves as official proof of the business's name, address, and EIN. It is important for businesses to secure this letter as it is often required for various legal, financial, and tax-related purposes, such as opening a bank account or filing business tax returns.