There are objects in the schema that are normally displayed under their parent objects (triggers, indexes, etc - see the screenshot).  It is often useful to see all such sub-objects in a single list (and therefore be able to synchronize them all with a single click), the Diff provides what we call 'summary collections'.  Foreign keys or other sub-object will be displayed under their parent tables and also in a single summary collection. In the picture below you can see how FOREIGN KEYs from all loaded tables are displayed in a single list as the "Foreign keys" summary collection:

This options page allows you to specify which types of schema objects should or should not be represented in their summary collections.