Users can define what we call 'linkableApps' in info.magnolia.editor.body.ContentEditorConfiguration#linkableApps. Default one's are assets and pages.
Once those are specified when user clicks on the link button, one will be seeing a choose dialog which contains the given selection of linkableApps.
Users have to choose from there which app they are interested at and they will be directed to tree view of the given app and simply select which content should be linked.