The distinction which I learned, which may very well be non-universal, was that /tmp is for temporary files which are tied to a particular process. These files can be removed as soon as the process exits, including on reboot. The /var/tmp directory is for temporary files which may need to outlast a given process, possibly across reboots, and should thus be cleared out much more sparingly based on the ages of the files.
Of course, not all processes follow this distinction.
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