If you have a cross reference pointing at a particular heading, and then you try to delete that heading, the system will prevent you from doing this - otherwise, you would end up with cross references pointing to non-existent locations. When this happens, a notification will popup telling you where the cross references are.

In the case below, we tried to delete 10. Test section, but cross references in 6.5.3 and 6.7.2 prevented this. Once the cross references are removed from those sections, we will be able to carry out the delete operation.