An example: You on Earth see events X and Y happening at Alpha Centauri and Proxima Centauri respectively. If the observers at Proxima Centauri didn't see event

An example: You on Earth see events X and Y happening at Alpha Centauri and Proxima Centauri respectively. If the observers at Proxima Centauri didn't see event X happening at Alpha Centauri before Y happened there, and the observers at Alpha Centauri didn't see Y happening at Proxima Centauri before X happened there. Then the events can be seen in either order by an external observer depending on where they are and how fast they move. If an observer at Alpha Centauri or Proxima Centauri did see the other event before their event happened, then they have a well defined order ('is inside the light cone').