6.5 What is a Debian conffile?

Conffiles is a list of configuration files (usually placed in /etc) that the package management system will not overwrite when the package is upgraded. This ensures that local values for the contents of these files will be preserved, and is a critical feature enabling the in-place upgrade of packages on a running system.

To determine exactly which files are preserved during an upgrade, run:

     dpkg --status package

And look under "Conffiles:".