A CSD file is a game backup file generated by the gaming service Steam that lets users downloading and managing Valve games. It contains the backup data related to games that can be used to restores a deleted game. Steam is only able to restore game from a CSD file if the game was completed downloaded, installed and patched through Steam. In order to restore the game from CSD file, use the option of Steam → Backup and Restore Gamesteam and select the file.
CSD File Format - More Information
The internal structure of CSD file format is not available publicly and its file format specifications are not available for developer’s reference. CSD files are accompanied by the CSM and ISS files which are also Steam game file formats.
Where to Find Steam Backup Files?
The entire Steam backup files can be found at the following locations: