1 Explanation: https://debian-handbook.info/browse/stable/sect.apt-get.html#sect.apt.priorities
2 Explanation: And man apt_preferences
3 Explanation: Installed packages get 100 priority, so this won't upgrade testing
4 Explanation: packages unless explicitly asked to. Just went middlish between 1-100
5 Explanation: Install with apt-get install package/testing. But if dependencies are
6 Explanation: needed, or need upgrading,
7 Explanation: apt-get -t testing package, setting testing to priority
8 Explanation: 990 just for that command.
9 Explanation: Use apt-cache policy to verify these settings.
11 Pin: release a=unstable
15 Pin: release a=unstable-updates
19 Explanation: Allow this testing package to automatically upgrade.