I am currently reading the book "Classical charge particle" by Fritz Rohrlich, and I struggle a lot with the appendix "space-like planes and Gauss's integral theorem".
He says "the world line of a particle determines uniquely its now-plane at every instant and as seen by any inertial observer". My question is basically : what is a "now-plane", and qualitatively, what am I suppose to understand from this sentence? (basically : why is it worth stating that?).