about file *.bak

No. I can tell you what I think it may be but that doesn't mean that I'm right because *.bak could be anything.

I think it is a backup copy of a file created by some program (could be a direct copy, ie. copy *.dat *.bak) but without knowing where the file is and/or it's exact name it's impossible to say.
 
as cromewell said, a .bak file is a backup of another file
Common bak file at created as backup to the old autoexec.bat, and config.sys files but in truth .bak could be a backup copy of anything
 
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