From the point of view of a client reading events from the server, events that result from that client's own requests arrive in the order in which it makes the requests. However, those events may be interspersed with events that result from other causes, such as user input or another client's actions. Furthermore, the client may buffer requests and the server may buffer events before actually transmitting them, so an event may arrive long after the client makes the request that generates the event. The point is that for the most part event processing in X is inherently asynchronous. The client cannot assume, for example, that a given input event was generated after a given client request just because the client read the event after it made the request.
Many events have timestamps that indicate when the server actually generated the events.
A client that depends on the temporal ordering of events must often examine these timestamps.