diff --git a/Design-Doc%3A-%60level%60-tool.md b/Design-Doc%3A-%60level%60-tool.md
index 1b6f4a0..3a5ec7f 100644
--- a/Design-Doc%3A-%60level%60-tool.md
+++ b/Design-Doc%3A-%60level%60-tool.md
@@ -107,4 +107,5 @@ Holds the list of active component attributes for each object.
    * Remember each Component Attribute is 2 bytes.
 For (Object Table Size):
    (Component Attribute Union Size) bytes: Component attribute settings
+      * FF FF = unused slot in this union
 ```
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