Here's how I understand it after a little test using a drawing called MYFILE.DWG:
AutoCAD will makes a periodic backup of the file if enabled. The interval depends on the SAVETIME variable. The location is "Automatic Save File Location" under the files tab in options.
Saving the file MYFILE.DWG, results in MYFILE.BAK in the same folder as the working file (if BAK files are enabled). The BAK file contains what the .DWG file was from the last save.
Autosave creates a file called MYFILE_1_1_NNNN.SV$ Where NNNN is some random (?) number. The original .SV$ file is renamed to .BAK when the next autosave is run and a new version will have the .SV$ extension is created.
The MYFILE_1_1_NNNN.SV$ file is deleted after the working file is saved by the user, The MYFILE_1_1_NNNN.BAK is not deleted even if the file is closed.
I set the save counter for two minutes for my test, but savetimes were random, never less than four minutes and once after over 10 minutes even though I did not perform a save myself!
If the user clicks "save" to write out to the actual working DWG file, I assume the savetime counter gets reset?
Sound about right? Any comments?