The sync type determines how the source and the destination are compared and can be changed in the Destination tab of a sync item’s settings.
Please note that source and destination will be compared each time a sync is run (in particular, each time the app is launched).
Due to a bug in macOS, if the source drive (or the destination drive 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 being unavailable. For SMB volumes, this issue will likely be fixed by Apple in an upcoming macOS release; 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 are deleted only if the option “Remove files not on source” is enabled.
Merge source and destination. Files that are only on one side are copied if they have been modified * after the last sync or deleted if they have been modified before the last sync: this way, deleted files are not going to be restored with the next sync (except for the first sync, where all unique files are copied). If a file exists on both sides and one of them is overwritten, any changes to the other file will be lost since no merging is done inside the files themselves.
The destination acts as a mailbox ingesting files from the source. As soon as a file has been successfully copied to the destination, it gets removed from the source.
* If the file is indexed by Spotlight and the “date added” attribute is available and indicates a date later than the last sync date, the file will be copied even if it wasn’t modified after the last sync, which is the desired behaviour. Since this attribute is not always available, caution is recommended when moving or renaming files managed by two-way syncs. If the attribute is not available and you want to add a file that has been modified before the last sync, you will have to copy it manually to both the source and destination or add it to the source while the sync is running in background, otherwise it could be erroneously deleted. You can also move a file to be deleted to the Add list and vice versa from the context menu in the confirmation window.
Please note that due to an issue in macOS 10.12 (Sierra) and later, the date added will not be set on renamed files, so that in two-way syncs they will appear in the delete list by default. In this case, please move any renamed files manually from the Delete list to the Add list from the context menu in the confirmation window. Renaming files while a sync item is watching the source in background is safe. One-way syncs and one-way moves are not affected.