# What is the catalog?

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 remember the previous state of your folders in order to detect deletions and conflicts.

If you are not using any options that require the catalog, no catalog is stored with the document. The catalog is also reset whenever you change a source folder, or change the action mode between backup and synchronization. If no catalog is present, and you have enabled options that require a catalog, you will have to run the document once first to build the catalog. (A message will appear in the window to remind you of this fact.) Your catalog-dependent options will then begin operating on the second run.