Each time you modify a file in your backup set, SpiderOak saves the changed file as a new 'version' rather than overwriting the existing file. The previous version of your file then becomes a historical version.
Historical versions only include the information that differs from the previous version; the rest of the file information deduplicates. Most historical versions take up little additional space in your account. For more information see Deduplication.
Documents that have historical versions saved will be followed by a number in parentheses which indicates how many historical versions SpiderOak has saved For example: file.doc (3)