diff --git a/versions/3.0.md b/versions/3.0.md
index 1d6a2d02a9..fa9a0d7749 100644
--- a/versions/3.0.md
+++ b/versions/3.0.md
@@ -1945,7 +1945,18 @@ An object to hold data types that can be consumed and produced by operations. Th
 
 Field Pattern | Type | Description
 ---|:---:|---
-<a name="definitionsName"></a>{name} | [Schema Object](#schemaObject) | A single definition, mapping a "name" to the schema it defines.
+<a name="definitionsName"></a>{name} | [Schema Object](#schemaObject) | A single definition, mapping a "name" to the schema it defines.  The following characters are legal in the `definitionsName`: `a-zA-Z0-9.-_\`.  All other characters, including extended ASCII characters, are not legal in the `definitionsName`.  The name may be used to namespace the definitions--it is the tooling vendor's job to ensure that the namespace is honored.
+
+Examples:
+
+```
+User
+User_1
+User_Name
+user-name
+my.org.User
+my\org\User
+```
 
 ##### Definitions Object Example