--- a/debian-apt.rst	Sun Jan 17 17:48:27 2021 +0200
+++ b/debian-apt.rst	Tue Feb 23 18:40:36 2021 +0200
@@ -120,6 +120,15 @@
 Check package versions and priority
 ===================================
 
+To show available attributes for package sources::
+
+  apt-cache policy
+  ...
+  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
+  ...
+
 ``/etc/apt/preferences``::
 
   Package: *