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.
Model vs View
Many users are confusing the difference between diagramming or drawing tools such as Microsoft Visio and modeling tools such StarUML or Rational Software Architect. First you need to understand a diagram is not a model.
Model or software model is a description of any aspect of a software system such as structure, behavior, requirement, and so on. A software model can be represented in textual, mathmatical or visual form. A Model element is a building block of a software model.
A Diagram is a visual geometric symbolic representation of a software model. A software model can be represented in one or more diagrams with different aspects. For example, a diagram can focus on class hierarchical structure while another diagram can focus on interaction between objects. Diagrams consists of view elements, which are visual representations of a model element.
A model element may have multiple corresponding view elements. A model element has its own data such as name, stereotype, type, etc. A view element just renders the corresponding model element in a diagram. View elements may exists multiple times in a diagram or in different diagrams. If the name of a model element changed, all corresponding view elements reflect the changes in their diagrams.
A fragment is a part of a project saved as a separate file with the extension name .mfj. Any element can be exported as a fragment, but typically UMLPackage, UMLModel, and UMLSubsystem are the candidates. Once a fragment is exported as a file, the fragment can be reused by importing in a project.
UML (Unified Modeling Language) is a general-purpose modeling language that could be used to express any kinds of software-intensive systems. In this reason, using UML for a specific domain or platform is not sufficient, so you may need to define UML Profile. StarUML provides UML profiles which can be used to expand UML. For example, UML profiles can be used for the following purposes.
Profiles for specific programming languages (C/C++, Java, C#, Python, etc.)
Profiles for specific development methodologies (RUP, Catalysis, UML Components, etc.)
Profiles for specific domains (EAI, CRM, SCM, ERP, etc.)