PMBOK, pg. 78 "Described in Section 12.2.3.2. Agreements are used to define initial intentions for a project. Agreements may take the form of contracts, memorandums of understanding (MOUs), service level agreements (SLA), letters of agreement, letters of intent, verbal agreements, email, or other written agreements. Typically, a contract is used when a project is being performed for an external customer."
I think of something like a JV where two companies agree to drill for oil with some sort of risk-sharing proposition. A project has not started, but the company has formally entering into negotiations and signed a deal. Happens all the time. Now, we need to do a project to procure the land, rigs, and oil field equipment. The JV agreement/contract is an input to the develop project charter. The procurement management processes would be formally invoked to procure the services of a drilling company and other necessary equipment.
The project charter would list the risk sharing agreement and other high-level terms, operating expenses, operating company, etc.
Hope this helps.