Properties describe the attributes of a class and consist of a name and a type. Regarding the name, please keep these two rules in mind[^1]:
- Property names must be unique within their owning class
- They may not be empty
But despite that, you're free to choose whatever you like as a name.
To specify your property's type, just double-or long-click on the string representing the type: A dropdown list will appear containing a list of all classes/interfaces defined in the diagram, together with some predefined standard data types. If you need another type not yet in the list, start typing its name and press enter to add it to the list automatically. Your newly added data type will then be available throughout your whole diagram.
If your property points to another class from your diagram, it updates automatically whenever the corresponding class changes. Accordingly, when you delete the class, the property also gets removed.
Another feature only available when your property references a class or interface is converting your property to an association and vice versa. For this, open the context menu on your property and choose the desired kind of association.
References and further readings