The catalog is a database stored inside your Synk document which contains information about the state of your source folders at the end of the last successful run. It contains a list of all of the files present, along with their modification dates.

The catalog is used by Synk to detect when an item has been deleted since the last run (for the Synchronize deletions setting) as well as to detect conflicts (for the Conflict Resolution settings)

The catalog is reset whenever you change a source folder, or change the action mode between the backup directions or synchronization.