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Aggregation is needed to let multiple SIP protocol clients, devices, endpoints, or services, all publishing on behalf of the same user, arrive at a single unified value for an arbitrary category. These different publishing SIP protocol clients of the user are not generally aware of one another, nor are they aware of the different data that the others publish.
For example, a service might publish a user's calendar state, while the user's physical device might publish its activity as a device state. Both of these publications affect the user’s overall presence status, but neither the service nor the device has all of the information that it needs to publish that overall status. The implication is that another processing entity, with access to both pieces of information and the logic to combine them, is necessary to produce the overall state. This other processing entity is chosen to be the server. The details of aggregation are described in section 3.8.