The unique ID is associated to the item, not to the user. Remember that the Data Adapter is completely shielded from the actual users subscribing to an item. It receives a subscription when the first user subscribed to an item, it receives an unsubscription when the last users subscribes to that item, and not notifications happen in the middle.

On the other hand, the user identity for each subscription and all the authorization logic is handled by the Metadata Adapter.

See ARI "Protocol.pdf" for a full description of the protocols used for both the Data Adapter and the Metadata Adapter.