diff --git a/docs/index.rst b/docs/index.rst
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -6,8 +6,9 @@
    :maxdepth: 2
    :caption: Contents:
 
-   package-loader-tutorial
    vcs-loader-overview
+   package-loader-tutorial
+   package-loader-specifications
 
 
 Reference Documentation
diff --git a/docs/package-loader-specifications.rst b/docs/package-loader-specifications.rst
new file mode 100644
--- /dev/null
+++ b/docs/package-loader-specifications.rst
@@ -0,0 +1,106 @@
+.. _package-loader-specifications:
+
+Package loader specifications
+=============================
+
+Release fields
+--------------
+
+Here is an overview of the fields (+ internal version name + branch name) used by each package loader, after D6616:
+
+.. list-table:: Fields used by each package loader
+   :header-rows: 1
+
+   * - Loader
+     - internal version
+     - branch name
+     - name
+     - message
+     - synthetic
+     - author
+     - date
+     - Notes
+   * - archive
+     - passed as arg
+     - ``release_name(​version)``
+     - =version
+     - "swh-loader-package:
+       synthetic revision message"
+     - true
+     - SWH robot
+     - passed as arg
+     -
+   * - cran
+     - ``metadata.get(​"Version", passed as arg)``
+     - ``release_name(​version)``
+     - =version
+     - =version
+     - true
+     - ``metadata.get(​"Maintainer", "")``
+     - ``metadata.get(​"Date")``
+     - metadata is intrinsic
+   * - debian
+     - passed as arg (eg. ``stretch/contrib/0.7.2-3``)
+     - ``release_name(​version)``
+     - =version
+     - "Synthetic revision for Debian source package %s version %s"
+     - true
+     - ``metadata​.changelog​.person``
+     - ``metadata​.changelog​.date``
+     - metadata is intrinsic. Old revisions have ``dsc`` as type
+   * - deposit
+     - HEAD
+     - only HEAD
+     - HEAD
+     - "{client}: Deposit {id} in collection {collection}"
+     - true
+     - SWH robot
+     - ``<codemeta: dateCreated>`` from SWORD XML
+     - revisions had parents
+   * - nixguix
+     - URL
+     - URL
+     - URL
+     - ""
+     - true
+     - ""
+     - None
+     - it's the URL of the artifact referenced by the derivation
+   * - npm
+     - ``metadata​["version"]``
+     - ``release_name(​version)``
+     - =version
+     - =version
+     - true
+     - from int metadata or ""
+     - from ext metadata or None
+     -
+   * - opam
+     - as given by opam
+     - "{opam_package}​.{version}"
+     - =version
+     - =version
+     - true
+     - from metadata
+     - None
+     - "{self.opam_package}​.{version}" matches the version names used by opam's backend. metadata is extrinsic
+   * - pypi
+     - ``metadata​["version"]``
+     - ``release_name(​version)`` or ``release_name(​version, filename)``
+     - =version
+     - "{version}: {metadata[​'comment_text']}" or just version
+     - true
+     - from int metadata or ""
+     - from ext metadata or None
+     - metadata is intrinsic
+
+using this function::
+
+    def release_name(version: str, filename: Optional[str] = None) -> str:
+        if filename:
+            return "releases/%s/%s" % (version, filename)
+        return "releases/%s" % version
+
+
+The ``target_type`` field is always ``dir``, and the target the id of a directory
+loaded by unpacking a tarball/zip file/...