diff --git a/README.rst b/README.rst
index 8c11fff..4a03596 100644
--- a/README.rst
+++ b/README.rst
@@ -525,13 +525,20 @@ candidate list further down.
   - The `namespace` is the "vendor" name such as "debian" or "ubuntu".
     It is not case sensitive and must be lowercased.
   - The `name` is not case sensitive and must be lowercased.
-  - The `version` is is the package version.
-  - `arch` is the `qualifiers` `key` for a package architecture
+  - The `version` is the version of the binary (or source) package.
+  - `arch` is the `qualifiers` `key` for a package architecture. The special
+    value `arch=source` shall identify a Debian source package (which
+    usually consists of a Debian Source control file (.dsc) and corresponding
+    upstream and Debian sources). In this case, `name` and `version` of the
+    source package shall be given (both may differ from binary package!), as
+    printed by `dpkg-query -f '${source:Package} ${source:Version} -W <pkg>`.
   - Examples::
 
         pkg:deb/debian/curl@7.50.3-1?arch=i386&distro=jessie
         pkg:deb/debian/dpkg@1.19.0.4?arch=amd64&distro=stretch
         pkg:deb/ubuntu/dpkg@1.19.0.4?arch=amd64
+        pkg:deb/debian/attr@1:2.4.47-2?arch=source
+        pkg:deb/debian/attr@1:2.4.47-2%2Bb1?arch=amd64
 
 - `docker` for Docker images