A BKS file is a Microsoft Works backup file that stores the last working state of a spreadsheet file. This saves the last valid state of the spreadsheet to be reloaded again in case the original document gets corrupted or cannot be opened due to any reason. BKS is similar to WPS file format that contains document content similar to Microsoft Word DOC document. Microsoft Works has been discontinued and BKS files are no more supported.
BKS File Format
BKS files are stored to disc as binary files and its file format specifications are not available publicly.