Irs Tax ID Number
Employers need both a federal tax ID FEIN, and a state employer ID number SEIN.
Also known as an IRS number, it is a unique 9 digit number (ex. 12-3456789) that the Internal Revenue Service and other federal agencies use to identify your business for many tax purposes, such as general business profit income tax and employment withholding tax.
All businesses need a federal tax ID number OR EIN ( federal employer identification number except sole owners that can use their social security number but who nevertheles may obtain a federal ID to use as a business tax ID number.
Corporations, partnerships, LLCs, employers, and, in some states, those businesses that are also getting a Seller's Permit are required to obtain a Federal Tax Id (FEIN) and a Business License. You can use a federal tax id number as your business id number.
Otherwise, you will need to use your social security number over the phone or the internet all the time for orders or other business identification reasons. And your don't want your social security out there if you want to protect your identity.