differences between two rsync commands

I'm following the postgresql documentation for upgrading a cluster of 2 machines - one primary and one secondary (with replication..). According to the docs, I need to upgrade only the primary and then I can run rsync on the primary to sync the secondary. The command that is mentioned in the docs : rsync --archive --delete --hard-links --size-only --no-inc-recursive primary_parent_of_old_data_dir primary__parent_of_new_data_dir secondary_remote_parent_of_old_data_dir I wanted to run the rsync command from the secondary, therefore I used the following command : rsync --archive --hard-links --size-only --verbose --human-readable --no-inc-recursive --delete root@primaryIP:primary_parent_of_new_data_dir secondary_parent_of_old_data_dir I wanted to ask what is the difference between the commands. Why do I need to specify 3 directories in the first command and not just two?