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  • Ahmed Abdi thinks this is interesting:

The project charter establishes a partnership between the performing and requesting organizations. In the case of external projects, a formal contract is typically the preferred way to establish an agreement. In this case, the project team becomes the seller responding to conditions of an offer to buy from an outside entity. A project charter is still used to establish internal agreements within an organization to assure proper delivery under the contract. The approved project charter formally initiates the project. A project manager is identified and assigned as early in the project as is feasible, preferably while the project charter is being developed and always prior to the start of planning. The project charter should be authored...

From

Cover of A Guide to the Project Management Body of Knowledge (PMBOK® Guide), Fifth Edition

Note

why use internal project charter if your buying the product from external company?