diff --git a/doc/flatpak-metadata.xml b/doc/flatpak-metadata.xml
index 9b2f68a3..77072689 100644
--- a/doc/flatpak-metadata.xml
+++ b/doc/flatpak-metadata.xml
@@ -344,6 +344,70 @@
+
+ [ExtensionOf]
+
+ This optional group may be present if the runtime is an extension.
+
+
+
+ (string)
+
+ The ref of the runtime or application that this extension
+ belongs to.
+
+
+
+ (integer)
+
+ The priority to give this extension when looking for the
+ best match. Default is 0.
+
+
+
+
+
+ [Extra Data]
+
+ This optional group may be present if the runtime or application uses
+ extra data that gets downloaded separately. The data in this group
+ gets merged into the repository summary, with the xa.extra-data-sources
+ key.
+
+
+ If multiple extra data sources are present, their uri, size and checksum
+ keys are grouped together by using the same suffix. If only one extra
+ data source is present, the suffix can be omitted.
+
+
+
+ (boolean)
+
+ Whether to mount the runtime while running the /app/bin/apply_extra
+ script. Defaults to true, i.e. not mounting the runtime.
+
+
+
+ (string)
+
+ The uri for extra data source X. The only supported uri schemes are
+ http and https.
+
+
+
+ (integer)
+
+ The size for extra data source X.
+
+
+
+ (string)
+
+ The sha256 sum for extra data source X.
+
+
+
+