Note 1 to entry: If a displacement four-vector ${\underset{\_}{\underset{\_}{x}}}_{\text{AB}}$ of two events A, B is a light-like vector, then in any inertial frame both events A, B can be connected by a signal with speed of light in vacuum.

Note 2 to entry: Owing to the nature of a four-vector, not all components of a light-like vector need to be zero.