This command compares the current file with a saved backup version of the same file. See: File Compare.
Select the backup file that contains a previously saved version, then click OK to show the differences. The number of backup files available depends on how many times you saved a file, and how long backup files are stored. The backup settings are controlled in the File Options dialog. See: File Options.
Every time you save a file for the first time in a session, normally Source Insight saves a copy of the original file in as a backup. A unique number is added to the file name so multiple versions of the file can exist in the backup directory.
Each project has its own backup directory. You can set it in the Project Settings dialog. See: Project Settings.