Project is a top-level element stored as a single file (.mdj). Modeling a software system requires describing multiple models because it is not enough to describe the system with a single perspective, so we typically make multiple models such as Use-Case Model, Design Model, Component Model, Deployment Model, or others in a Project. Typically Project is organized as a set of UMLModels, UMLPackages, or UMLSubsystems. If you want to know more about UML Elements, please refer to OMG UML Specification.