The actual storage structure depends on which Driver each Storage is based on. When using the local file system Driver provided by the TYPO3 CMS Core, a Storage will correspond to some existing folder on the local storage system (e.g. hard drive). Other Drivers may use virtual structures.

By default, a Storage pointing to the fileadmin folder is created automatically in every TYPO3 CMS installation.

Processed files

Inside each Storage there will be a folder named _processed_ which contains all resized images, be they rendered in the frontend or thumbnails from the backend. The name of this folder is not hard-coded. It can be defined as a property of the Storage. It may even point to a different storage.

Defining a location for processed files

Editing a File Storage to define a location for processed files

Migrated files

When upgrading from a pre-FAL installation (i.e. a TYPO3 CMS version older than 6.0), files will have been moved from various locations (but generally the uploads folder and its sub-folders) to a folder named _migrated in the default Storage (or other Storages if you had several). Such a folder may also be used by custom processes provided by extensions.

This folder contains active files from your older TYPO3 CMS installation. It should not be deleted unless you are sure that you are not using any of these files anymore. It would be advisable to move all files out if this folder over time and into a more explicit structure.