Files created in File Backup Data Folder

I do not understand what files are created in the My Documents Backup.  The files I am refering to are “fbfFilesXXX”.  My settings are to backup ALL file types.  There are very few changes in regular documents and such but a lot of email; activity.  The My Documents Backup is scheduled to run EVERY night.  Some nights it creates 5 separate folders, other nights it creates just a couple and some nights it does not create any.  It NEVER automatically removes any files (like it does for the My Computer Backups) so how do you manage these files?  When is it safe to remove OLD files to manage the available space on your destination drive and how do you know which ones to remove?  I guess I need an explanation of how the files are created initially and each night what is created in the subsequent files.  I have months worth of files and have no idea what is in them or how to manage them.