/etc/apt/preferences.d/

• https://manpages.ubuntu.com/manpages/xenial/man5/apt_preferences.5.html

See also

• /etc/apt/, /etc/apt/sources.list.d/, deb, sources.list
• APT: wajig, apt-cache, add-apt-repository PPA, apt [ install | remove |purge | show | list | changelog | update | upgrade], do-release-upgrade, apt-mark, apt full-upgrade, sources.list, APT Automatic updates, unattended-upgrade, apt-transport-https, apt-get update, /etc/apt/, /etc/apt/apt.conf.d/50unattended-upgrades, /etc/apt/apt.conf.d/, /etc/apt/apt.conf.d/20auto-upgrades, apt-file, apt-utils