Project Tree Selection Behavior
The selected item in the Project Tree can affect the possible actions you can perform. For example if you want to add a locator to a specific class, you need to first select a class. Similarly, if you want to add a validation rule, you also need to have the desired class selected.
This behaviour is typical for a hierarchy, but the Project Tree selection behaviour goes one step further.
This behavior does not however, apply to parent classes. If you select an item within a child class, only the child class is indirectly selected. In order to perform an action on the parent class, it or one of its child elements other than a class should be selected. This restriction ensures that you do not inadvertently add an item to the parent class when it is meant for a child class.
For example, you have just finished configuring an Advanced Zone Locator, and want to add some fields that correspond to that locator. You can do this by keeping your current Project Tree selection and going directly to the Ribbon Design tab, and clicking "Field". This is possible because the Advanced Zone Locator and the field you need to create are both child items of the same class, and the class selection is implied when the locator is selected.