I'm referring to the docs at "Packaging Guidelines for AppData Files"
<https://docs.fedoraproject.org/en-US/packaging-guidelines/AppData/>

The AppStream project has made a number of changes since these Fedora guidelines were written, and the conflicting information confused me at first, as I looked up how to add AppStream metadata to a new package I'm working on...

* In the Fedora guidelines, the second sentence
  ("Installed .appdata.xml files MUST follow the AppData specification page.")
  links to 
  <http://people.freedesktop.org/~hughsient/appdata/>
  but this URL now redirects to
  <https://www.freedesktop.org/software/appstream/docs/chap-Quickstart.html#sect-Quickstart-DesktopApps>

* As far as I can tell, the AppStream specification no longer
  uses the filename to distinguish between applications and
  addons, and all AppStream metadata files should now be named
  .metainfo.xml. Files for GUI applications are no longer named
  .appdata.xml.
  See <https://www.freedesktop.org/software/appstream/docs/chap-Quickstart.html#sect-Quickstart-DesktopApps>

* The example .appdata.xml file in the Fedora guidelines is
  therefore also out of date. The current AppStream spec,
  for example, states that the component type should be
  "desktop-application" instead of just "desktop".

Unless I'm misunderstanding something, the Fedora guidelines here will need to be rewritten somewhat.