Yes, a Federal Employer Identification Number (FIN or UN) is pretty much a must for any Illinois Series LLC. You need a UN if you want to hire staff, use taxes like S Corporation, or obtain business licenses in Illinois. Most banks also expect your LLC series to have a UN to open a bank account. You can obtain an employer identification number via the IRS website, fax or mail. It is also important to indicate the obligations, rights and obligations of each member and manager in each LLC series if it differs from the Master. Although an LLC offers the limited liability of a corporation, this unit is easier to form. A Series LLC offers each company protection against commitments made by other companies within the series.