Three numbers, three jobs
An SSN is primarily an individual's Social Security number. An ITIN is an IRS-issued taxpayer identification number for certain individuals who need a U.S. taxpayer identification number but are not eligible for an SSN. An EIN is an IRS-issued identification number for employers and many other business or entity tax accounts.
The mistake to avoid is treating the three as interchangeable. A form asking for the taxpayer identification number of an entity may require an EIN, while a form asking for the owner or responsible party may require an individual's SSN or ITIN.
Why a business may encounter more than one
A single-member LLC can have an EIN while its owner still uses an SSN or ITIN on forms that ask for the individual owner. A corporation will generally use its EIN for federal corporate filings while officers and shareholders continue to have their own individual taxpayer IDs. Payroll records also connect the employer's EIN with each employee's SSN.
Form W-9 is a good example
Form W-9 asks a U.S. payee for a taxpayer identification number and certifications. The correct TIN depends on the payee's federal tax classification. That is why a company should not guess a contractor's tax number from an invoice; request a properly completed W-9 and retain it.
Practical rule
Read the exact field label and the instructions for the form. Use the EIN where the business entity's tax account is being identified, and use the individual taxpayer ID where the individual is the taxpayer or responsible person being identified. If a bank, marketplace, or customer portal asks for a tax ID without explaining which one, verify what taxpayer or entity they are trying to match before submitting the number.