The sync type determines how the source and the destination (or, more generally, the bases) are compared and can be changed in the Destination tab of a sync item’s settings. The bases are compared each time a sync is started.
Warning: Due to a bug in macOS, if the source drive (or any of the drives for a two-way sync) is connected remotely (e.g. via FTP or SMB) and it becomes unavailable during the scan phase, files will automatically be marked for deletion, since macOS doesn’t differentiate between a file being deleted or just unavailable. For SMB volumes, this issue could be fixed by Apple in the future; unfortunately they have no plans to improve support for FTP volumes. In the meantime, it is always recommended to manually confirm the changes for sync items involving a remote volume.
The destination acts as a backup of the source, which is never changed by a sync. This is the default sync type. Files that exist only on the source are always copied to the destination. Files that already exist on the destination are overwritten according to the overwrite policy chosen in the Destination tab of the settings. Files that exist only on the destination can be deleted by enabling the option “Remove items not on source”.
Merge all the bases. On the first sync, all unique files are copied. Afterwards, files that are only on one side are copied or removed if they have been added or deleted after the last sync, respectively. If a file is modified (the modification date or file size has changed since the last sync), it overwrites the counterpart on the other side. If the same file has been modified on both sides, it is considered a conflict and an error is logged. The conflict can be solved by manually copying the file that you want to keep to the other side.
The destination acts as a mailbox ingesting files from the source. When a file has been successfully copied to the destination, it is removed from the source.