Use the --help or -? flag to see the complete list of options. Option flags may appear before, after, or between the ORIGIN and REPLICA arguments.
Both databases may be "live" while this utility is running. Other programs can have active connections to the databases on either end while this utility is running. Other programs can write to ORIGIN and can read from REPLICA while this utility runs.
REPLICA becomes a copy of a snapshot of ORIGIN as it existed when the sqlite3-rsync command started. If other processes change the content of ORIGIN while this command is running, those changes will be applied to ORIGIN, but they are not transferred to REPLICA Thus, REPLICA ends up as a fully-consistent snapshot of ORIGINAL at an instant in time.
While sqlite3-rsync is running, REPLICA is read-only. Queries can be run against REPLICA while this utility is running, just not write transactions.