UML diagrams are completely necessary for the creation of the test cases. That is because every message of the represents an action that will be carried on in the test (click or type, for instance).

In the next paragraphs, it is explained how can an UML diagram be created:

When logged in the tool, the user can see a menu in the left side (see First steps in TAST for further info), in this menu the user will click on the button “Modeling”. After this, there will open a window where the user will select the domain and project where the test case will be included in. NOTE: the projects and domains need to be created before the creation of the test case and, also, the used needs to be assigned to the domain and project. Otherwise, the user will not be able to see neither the project-domain, nor the test cases that might have already been created. For more info about the creation of the Domain and user, please check Domains and Projects page. Once in the Modeling page, the user can proceed in three ways:  Selecting and modifying one of the test cases already created.  Creating a new test case.  Importing a test case created with any other UML Design tool.