clarify uniqueness

Author: XVincentX

graph/patterns/alternate-key.md

@@ -8,7 +8,7 @@ _The Alternate Key Pattern provides the ability to query for a single, specific
 
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-All entities in our system are identified by an [UUID (Universally Unique Identifier)](https://en.wikipedia.org/wiki/Universally_unique_identifier) - which guarantees uniqueness. Often though, that same resource can also be uniquely identified by an alternative, more convenient attribute that provides a better developer experience.
+The resources exposed in Graph are identified by an [UUID (Universally Unique Identifier)](https://en.wikipedia.org/wiki/Universally_unique_identifier) - which guarantees uniqueness inside the same resource type. Often though, that same resource can also be uniquely identified by an alternative, more convenient attribute that provides a better developer experience.
 
 Take a look at the `user` resource: while the `id` remains a perfectly valid way to get the resource details, the `mail` address is also an unique attribute that could be used to identify it.