When a new version of Slax is released, you would need
to download whole new iso image. Even if it's under 200
MB, it could be hard task for some people with slow
connection. These are special diff files, which contain
only the differences between older and current Slax,
thus they are smaller than the original.

You can create new ISO file from the old one if you have bsdiff patch.
Just put slax-OLD_VERSION.iso and Slax-OLD_VERSION-to-NEW_VERSION.bsdiff
in the same directory, and run:

bspatch slax-OLD_VERSION.iso slax-NEW_VERSION.iso slax-OLD_VERSION-to-NEW_VERSION.iso.bsdiff

The same applies for TAR extension, of course.
And also, there are bsdiff files to make TAR from your ISO
and ISO from your TAR.

Download bspatch for windows or linux, both require gzip2.
Get them from here: ftp://ftp.slax.org/bsdiff-binaries/