Snapshot.md

@@ -10,8 +10,8 @@ It's the only top level element currently used by OFMX.
 ```
 
 * `OFMX-Snapshot`<sup>[XT]</sup> – Snapshot root
-  * `xsi` – Always `http://openflightmaps.org/schema/4.5-1/OFMX-Snapshot.xsd`
-  * `version` – Always `4.5-1`
-  * `origin` – Source of the data contained herein
-  * `created` – Data creation date and time
-  * `effective` – Data effective after this date and time
+  * `xsi`<sup>[M]</sup> – Always `http://openflightmaps.org/schema/4.5-1/OFMX-Snapshot.xsd`
+  * `version`<sup>[M]</sup> – Always `4.5-1`
+  * `origin`<sup>[M]</sup> – Source of the data contained herein
+  * `created`<sup>[M]</sup> – Data creation date and time
+  * `effective`<sup>[M]</sup> – Data effective after this date and time
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