Explanation: https://debian-handbook.info/browse/stable/sect.apt-get.html#sect.apt.priorities
Explanation: And man apt_preferences
Explanation: Installed packages get 100 priority, so this won't upgrade testing
Explanation: packages unless explicitly asked to.
Explanation: Install with apt-get install package/testing. But if dependencies are
Explanation: needed, or need upgrading,
Explanation: apt-get -t testing package, setting testing to priority
Explanation: 990 just for that command.
Explanation: Use apt-cache policy to verify these settings.
Package: *
Pin: release a=testing
Pin-Priority: 50

Package: *
Pin: release a=testing-updates
Pin-Priority: 50

Explanation: Allow this testing package to automatically upgrade.
Package: unison
Pin-Piority: 100