Drawing backup files are typically created every time that you manually save a .dwg file. By default the file will be saved in the same location as the .dwg and will have the same name as the drawing but with a .bak extension, such as, site_topo.bak. A backup file is an exact copy of the drawing file prior to the last save. As such, backup files are always one version older than the currently saved drawing. Only one backup file is retained at a time so newly created backups will always replace older backups of the same name.