- When you open a .stu file and make modifications, you save the current file.  Two files are actually created:

- a .BAK file which is a copy of the .STU file that you  had opened.
- a new .STU file with the modifications
- If you make several saves during the modifications, the content of the .BAK file is always equal to the n-1 save
- This file can be useful in the event of a crash. You are able to retrieve the previous application easily modifying the .BAK type of this file in .STU type