Cross-border contracts may involve multiple organisations operating under completely separate legal regimes. The main contracting parties might be headquartered in two different countries, and the contract's performance could even take place in a third country, further complicating issues.
Under international law, a treaty is any legally binding agreement between states (countries). A treaty can be called a Convention, a Protocol, a Pact, an , etc.; it is the content of the agreement, not its name, which makes it a treaty.
International contracts are legally binding agreements between parties who are based in separate countries. As with any contract, it will require the parties to do or refrain from doing particular actions.