Chapter 19 Troubleshooting : ­Troubleshooting tools : FortiOS diagnostics : Finding object dependencies
  
Finding object dependencies
An administrator may not be permitted to delete a configuration object if there are other configuration objects that depend on it. This command identifies other objects which depend on or make reference to the configuration object in question. If an error is displayed that an object is in use and cannot be deleted, this command can help identify the source of the problem.
Another use is if you have a virtual interface with objects that depend on it, you need to find and remove those dependencies before you delete that interface.