Replication actions can be configured to send updates on a schedule or continuously. The replication update process itself is the same in both cases. This property only controls the interval.
Because continuous replication actions send updates as frequently as possible, they result in sending a constant stream of all filesystem changes to the target system. For filesystems with a lot of churn (many files created and destroyed in short intervals), this can result in replicating much more data than actually necessary. However, as long as replication can keep up with data changes, this results in the minimum data lost in the event of a data-loss disaster on the source system.
Continuous replication is still asynchronous. ZFS Storage Appliances do not currently support synchronous replication, which does not consider data committed to stable storage until it's committed to stable storage on both the primary and secondary storage systems.