Explanation: Debian jessie
Package: *
Pin: release o=Debian,n=jessie
Pin-Priority: 990

Explanation: Debian stretch
Package: *
Pin: release o=Debian,n=stretch
Pin-Priority: 1

Explanation: Debian sid
Package: *
Pin: release o=Debian,n=sid
Pin-Priority: 1

Explanation: Debian fallback
Package: *
Pin: release o=Debian
Pin-Priority: -10