Lecture: Logging Basics - reference to Replication component which isn't discussed yet

The trainer states “For some of these Log components, there are sub-components as well. Remember, there were 3 different components for replication. (1) standard Replication Component, (2) Heartbeat component, and (3) Rollback component”

Nothing like this has been discussed in any of the previous Lectures to be remembered, so not sure if you have any Lectures missing here??