When you are given a new server or workstation to administer, you generally want to find out quickly what is installed and what is special about this computer. One way to find out is to look at all the package which has been added to the system, especially those which are not part of the original linux distribution.
This is the later that is achieve by this functionality. It extract the packages which are not member of the distribution. It does this by comparing the vendor and distribution tags of the package. All packages which are not matching the current distribution are shown. For each package, the package name, version, vendor and distribution are shown.