I still don't have a clear reply to my question.
I have a folder on desktop called "My DVD". I copy a video file to it and the back up TM to my HD. After backup I delete the file to free space in my MBP. I back up TM again. Now, how can I ensure that I will find a copy of this file in my TM HD for the longest period of time ? Sometimes, when I go back to my backups from January, I find some files there, but at other times, I cannot find a file that I copied into MY DVD just last week. This has something to do with how TM chooses backups to keep and I am asking if anyone know how time machine selects backups to preserve?