We have seen that if several observers watch two events, labeled A and B, one of them may say that event A occurred first but another may claim that it was event B that did so. What would you say to a friend who asked you which event really did occur first?

If two observers are in relative motion, in general they do not agree on whether two events at different locations are simultaneous. If one observer finds the two events to be simultaneous, the other does not. Thus, each frame of reference has its own particular time.
If the observer and the event occur in the same frame, then the two events will measure simultaneously at the same time by the observer.
Now consider the two events occur in the Sʹ frame and the observer in S frame. If the Sʹ frame including events moves with speed u, then we cannot measure the two events simultaneously. Clearly the observer will observe the event A first then the event B.