For Screenwriter 6 (Mac & Windows) and Screenwriter 6.5 (Mac) there are two kinds of backup files that are produced: Timed Backups (.tmb), and Previous Versions (.bk). The two are very different, and it's important to understand the difference.
ALWAYS SAVE YOUR DOCUMENT AS YOU WRITE: Before we get into the differences -- we must emphatically recommend that you regularly use the FILE > SAVE command to save your changes as you write. Since it only takes 1-2 seconds to save, and usually a single keystroke (COMMAND+S on the Mac, CONTROL+S on Windows), it should be convenient for you to save.
Timed Backups: The .tmb Timed Backup file is NOT the same as saving the document -- it is an emergency backup file that is created every few minutes (usually every 5 minutes) just in case your computer loses powerScreenwriter crashes. If you crash, Screenwriter usually recognizes there was a problem, and gives you the opportunity to open the .tmb -- which will be the exact state of your screenplay/play/novel document at the time the Timed Backup was created.
Previous Version Backups: The .bk Previous Version backup files represent the version of your script PRIOR to the last time you saved the document using the FILE > Save command. A preset number of .bk files are saved, and that preset number can be set in Screenwriter's FILE Preferences (Mac) or STARTUP / FILES Options (Windows).
You can also set the location where Previous Version files are stored. We strongly recommend selecting the FIRST radio button, "A subfolder named, "Previous Versions". This ensures that all backups of your specific Screenwriter document are located in a folder called "Previous Versions" which is also located wherever you document has been saved.
IMPORTANT: if you choose to open a Timed Backup .tmb or a Previous Version Backup .bk file, make sure that the first time you save, you are saving to a document with a .mmsw or .mmx file extension -- in other words, a script document. Do not continue to work and save in the .tmb or .bk file, as this can lead to confusion later on.