Configuration files: an important assumption debian takes

It took me more than expected (and in fact someone found it for me) to explain why a file installed by a debian package and not listed inside debian/conffiles was handled as a standard configuration file.

The file is installed in a subdirectory of /etc/, so intuitively you may assume it must be considered as a configuration file, although this wasn't clearly stated in a document until I read this from Debian New Maintainer's Guide , chapter 5:

Since debhelper V3, dh_installdeb(1) will automatically flag any files under the /etc directory as conffiles, so if your program only has conffiles there you do not need to specify them in this file. For most package types, the only place there is (and should be conffiles) is under /etc and so this file doesn't need to exist.

The conclusion is that if you install a file in a subfolder of /etc/, you don't need to list it inside debian/conffiles.