In the conclusion of Time, Clocks, and the Ordering of Events in a Distributed System, Leslie Lamport states that: the concept of “happening before” defines an invariant partial ordering of the events in a distributed multiprocess system.

According to a stackoverflow post, Jacob Baskin states that an invariant is a property of the program state that is always true. Tieing that together with the original question that I had asked in the previous paragraph, I think what Leslie is trying to say is that because of the happening order event, we know that [in a distributed system] events will be always be partially ordered — not totally ordered.