So this is my apt policy:
apt policy Package files: 100 /var/lib/dpkg/status release a=now 400 http://deb.debian.org/debian bullseye/main amd64 Packages release o=Debian,a=testing,n=bullseye,l=Debian,c=main,b=amd64 origin deb.debian.org 500 http://deb.debian.org/debian buster-updates/main amd64 Packages release o=Debian,a=stable-updates,n=buster-updates,l=Debian,c=main,b=amd64 origin deb.debian.org 500 http://security.debian.org/debian-security buster/updates/main amd64 Packages release v=10,o=Debian,a=stable,n=buster,l=Debian-Security,c=main,b=amd64 origin security.debian.org 500 http://deb.debian.org/debian buster/main amd64 Packages release v=10.6,o=Debian,a=stable,n=buster,l=Debian,c=main,b=amd64 origin deb.debian.org 450 http://ftp.debian.org/debian buster-backports/main amd64 Packages release o=Debian Backports,a=buster-backports,n=buster-backports,l=Debian Backports,c=main,b=amd64 origin ftp.debian.org Pinned packages:
Taking remmina as an example package which I installed from testing apt policy shows:
apt policy remmina: Installed: 1.4.8+dfsg-2~bpo10+1 Candidate: 1.4.8+dfsg-2~bpo10+2 Version table: 1.4.8+dfsg-2 400 400 http://deb.debian.org/debian bullseye/main amd64 Packages 1.4.8+dfsg-2~bpo10+2 450 450 http://ftp.debian.org/debian buster-backports/main amd64 Packages *** 1.4.8+dfsg-2~bpo10+1 100 100 /var/lib/dpkg/status 1.3.3+dfsg-2 500 500 http://deb.debian.org/debian buster/main amd64 Packages
apt upgrade (actually
apt full-upgrade, since there is some conflict) proposes to upgrade remmina.
As documented here (in German, section "Automatische Aktualisierungen", since the same section does not exist in the english version), choosing a pinning value of 200 (or like me 400) is the recommended way to get updates for backport packages, however, this contradicts (IMO) with the apt documentation:
100 < P <=500 causes a version to be installed unless there is a version available belonging to some other distribution or the installed version is more recent.
So, as far as I understand, remmina should not be upgraded since it is available in some other (stable, which is even the target release) distribution. Additionally, (it is not documented that way) but I would understand if the backports version is preferred because it is way newer, but then this would apply to most packages installed on my system. However, only the remmina package is proposed to be upgraded. Where am I wrong?