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: Less than 0 won't install package unless you specify the archive.
Explanation: This is good, so you never just search for a package and install
Explanation: it without knowing it's from a different archive.
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=unstable
Pin-Priority: -20

Package: *
Pin: release a=unstable-updates
Pin-Priority: -20

Package: *
Pin: release a=experimental
Pin-Priority: -40