By default, a sync item consists of two bases (a source and a destination).
You can define additional bases in the General tab of the sync settings by clicking on Add more bases and then on the + button below the list of bases.

One-way sync / One-way move
Multiple sources are virtually merged and mirrored to all the destinations.
Files on different sources with the same name but with different modification dates or sizes are considered a conflict, causing an error to be logged.
A file on the destinations is removed only if it is not present anymore on any of the sources.
Note: multiple sources are meant to bring files spread between different directories or volumes together into a single directory. If instead you have different directories and want to sync each one into its own destination directory, it is recommended that you either:
- create separate sync items or
- select the common parent directory as the source, exclude it and then include only the desired subdirectories (see Filters).
Two-way sync
In a two-way sync, the bases are always virtually merged and files that have changed on a base are mirrored to all other bases, hence there is no distinction between source and destination. In particular, between syncs, a file may be modified only on one base or it will be considered a conflict, causing an error to be logged. You can read more in Sync types.