According to SR, relativity of simultaneity is a concept that distant simultaneity is not absolute, but depends on the observer's reference frame.
Now it is impossible to say that two distant events accur at the same time if those events are separated in space.
However, if the events are casually connected, precedence order is reserved in all frames of reference.
Now to say that they are not casually connected, we must assume that the time between A, B, and the time between B, C (and the time between A,C) is less then the distance between them divided by c. Why do we need that?
We need to assume that because in this case according to SR, there is no way that information can reach from A to B inbetween the two happenings. If you send a photon from A at the time when A happens, this photon will only reach B after B already happened. A cannot influence B.
Events A,B,C will occur in different order depending on the relative motion of the observer.
The white line represents a plane of simultaneity. In the first picture, the events happen at the same time from the observer's frame. The white line moves upwards.
If the observer starts moving with o.3c, the events from his frame will not seem to be simultaneous, C will happen first.
If the same observer is moving in the opposite direction, with -0.5c, then A will seem to happen first.
This is only true, if the events are not casually connected.