Always On Availability Group – Select Data Synchronization

Select “Automatic seeding” (default) if you have not created the database on the secondary replica and would like this wizard to create the database on the secondary replica based on the same setting as the primary replica. Use this option, if this is a small database.

Select “Join only” if you already have already created the database on the secondary replica using the backup and logs. Use this option, if you have a large database.

Sources:

https://docs.microsoft.com/en-us/sql/database-engine/availability-groups/windows/select-initial-data-synchronization-page-always-on-availability-group-wizards?view=sql-server-ver15