When you save a workshared project, Revit creates a directory of backup files. In that directory, backup files are created each time a user synchronizes with central, or saves a local copy of the central model. Successive backups share as much element information as possible; therefore, they are incremental rather than equal in size to the entire project.
You can roll back either a central model or a local model of the project. For example, you may want to roll back to a prior version of the project, if changes made after a certain date are deemed incorrect or inappropriate. You can also save a prior version as a new project model.
When you roll back a model, all later versions in the backup directory are lost. In addition, you lose all information on workset ownership, borrowed elements, and workset editability. You must coordinate with team members to reassign workset and element ownership.
The backup folder for the central model contains files that store information about editability and ownership status (permission information). It also contains various DAT files and a worksharing log file (.slog) that provides progress information on operations (such as synchronize with central) to the Worksharing Monitor.
Revit stores central model backup information in a folder called [Centralfilename]_backup. Do not delete or rename any files in this folder. If you move or copy the project model, be sure the central model backup folder stays with the project model. If you rename the project model, rename the backup folder appropriately.
You can control the number of backups retained. For more information, see Specifying the Number of Backup Files.