在线状态实体是其在线状态信息被关注的实体。
A presentity is an entity whose presence information is of interest.
既然已经了解了如何发布在线状态实体信息以及如何以同步方式检索信息，也应该知道可以采用异步方式检索此信息。
Now that you've seen how to publish presentity information, and how to retrieve the information synchronously, you should know that there is also the option to retrieve the information asynchronously.
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