Operating Agreement is an internal document set by the company members that contains provisions for critical items and rules that run the company.
The Operating Agreement defines each member or manager's rights, powers and entitlements. This includes capital accounts, membership interest, distributions of profit and allocated tax responsibility, just to name a few.
Many states in the United States require an LLC to have an Operating Agreement. In those states that have no such requirement and LLC operating without an Operating Agreement are governed by the State's default rules contained in the relevant statute and developed through state court decisions.
And finally, in single member LLCs an Operating Agreement is a declaration of the structure that the member has chosen for the company and is sometimes used to prove in court that the LLC structure is separate from that of the individual owner and thus necessary so that the owner has documentation to prove that he or she is indeed separate from the entity itself.
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