Seems to me that the Illinios statutes say may and not must.
Sec. 15-5. Operating agreement.
(a) All members of a limited liability company may enter into an operating agreement to regulate the affairs of the company and the conduct of its business and to govern relations among the members, managers, and company. To the extent the operating agreement does not otherwise provide, this Act governs relations among the members, managers, and company.
(c)In a limited liability company with only one member, the operating agreement includes any of the following:
(1) Any writing, without regard to whether the
writing otherwise constitutes an agreement, as to the company's affairs signed by the sole member.
(2) Any written agreement between the member and the
company as to the company's affairs.
(3) Any agreement, which need not be in writing,
between the member and the company as to a company's affairs, provided that the company is managed by a manager who is a person other than the member.
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