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.
5 Explanation: Less than 0 won't install package unless you specify the archive.
6 Explanation: This is good, so you never just search for a package and install
7 Explanation: it without knowing it's from a different archive.
8 Explanation: Install with apt-get install package/testing. But if dependencies are
9 Explanation: needed, or need upgrading,
10 Explanation: apt-get -t testing package, setting testing to priority
11 Explanation: 990 just for that command.
12 Explanation: Use apt-cache policy to verify these settings.
14 Pin: release a=unstable
18 Pin: release a=unstable-updates