This folder is used for files that are backup using the 
FileBackup() function of X-Setup. 

X-Setup will automatically append .001.xsb, .002.xsb etc. to 
that file. For example, if the file "MSDOS.SYS" is backed up 
several times, the first file will be named "MSDOS.SYS.001.xsb",
the second "MSDOS.SYS.002.xsb" and so on.

Therefore: the higher the number, the newer the file.