Intro
A form of Reliable Broadcast. Guarantees that if one message is broadcast before another, it is delivered first.
Implementation
Causal order is not about time, but potential Happened-Before Relationships. Thus, Vector Clocks are used to determine this relationship.
Thus, if two messages are concurrent with one another, there is zero guarantee on what message is delivered first in the broadcast. But, if two messages are causal, then there is a guarantee.