If a model has its design data defined in a data dictionary, you can click on the icon in the lower-left corner of the model window to open the data dictionary in the Model Explorer. And you can see Design Data under the data dictionary inside the Model Hierarchy pane.
To identify which objects from the data dictionary are not used by the model, right-click Design Data and select Find Unused Variables. You will be asked to Update diagram. After that, the Model Explorer displays unused objects from the data dictionary.
Please refer the documentation page on Finding Unused Workspace Variables to learn more about the aforementioned steps.
To learn more about using data dictionaries in Simulink models, please refer to the example model on Using a Data Dictionary to Manage the Data for a Fuel Control System.
