Add some clarifying text about identifying attributes

Signed-off-by: Doug Davis <[email protected]>

cloudevents/spec.md

@@ -50,10 +50,10 @@ OPTIONAL for both the [Producer](#producer) and [Consumer](#consumer) of a
 message to support that feature. In other words, a producer can choose to
 include that feature in a message if it wants, and a consumer can choose to
 support that feature if it wants. A consumer that does not support that feature 
-is free to take any action it wishes, including no action or generating an error, as
-long as doing so does not violate other requirements defined by this specification.
-However, the RECOMMENDED action is to ignore it. The producer SHOULD be
-prepared for the situation where a consumer ignores that feature. An
+is free to take any action it wishes, including no action or generating an
+error, as long as doing so does not violate other requirements defined by this
+specification. However, the RECOMMENDED action is to ignore it. The producer
+SHOULD be prepared for the situation where a consumer ignores that feature. An
 [Intermediary](#intermediary) SHOULD forward OPTIONAL attributes.
 
 ### Terminology
@@ -271,6 +271,12 @@ and the event data will be serialized. For example, in the case of a JSON
 serialization, the context attributes and the event data might both appear
 within the same JSON object.
 
+Attributes are often used for identification purposes. While any particular
+attribute definition might include constraints on its value, in general this
+specification does not mandate how those identification attributes are
+constructed. For example, it might be a singleton (such as a name), or
+it could be a composite made up of multiple identifying sub-values.
+
 ### REQUIRED Attributes
 
 The following attributes are REQUIRED to be present in all CloudEvents: