diff --git a/docs/extrinsic-metadata-specification.rst b/docs/extrinsic-metadata-specification.rst
--- a/docs/extrinsic-metadata-specification.rst
+++ b/docs/extrinsic-metadata-specification.rst
@@ -182,3 +182,49 @@
 by the storage.
 Unifying these various formats into a common language is outside the scope
 of this specification.
+
+Artifact metadata
+^^^^^^^^^^^^^^^^^
+
+In addition to origin metadata, the storage database stores metadata on
+all software artifacts supported by the data model.
+
+This works similarly to origin metadata, with one major difference:
+as extrinsic metadata is given on an artifact within a given context,
+this context is stored along the metadata itself.
+
+For example, two origins may develop the same file independently;
+so authorship information on that file is only valid within an
+``origin`` context.
+
+Therefore, for each artifact type ``<X>``, there are two endpoints
+to manipulate metadata associated with artifacts of that type:
+
+* Adding metadata::
+
+      <X>_metadata_add(id, context, discovery_date,
+                       authority, fetcher,
+                       format, metadata)
+
+
+* Getting all metadata::
+
+      <X>_metadata_get(id, context,
+                       authority,
+                       after, limit)
+
+
+definited similarly to ``origin_metadata_add`` and ``origin_metadata_get``,
+but with an extra ``context`` (argument when adding metadata, and dictionary
+key when getting them) that is a dictionary with keys
+depending on the artifact type ``<X>``:
+
+* for ``snapshot``: ``origin`` (an URL) and ``visit`` (an integer)
+* for ``release``: those above, plus ``snapshot`` (a ``sha1_git``)
+* for ``revision``: all those above, plus ``release`` (a ``sha1_git``)
+* for ``directory``: all those above, plus ``revision`` (a ``sha1_git``)
+  and ``path`` (a byte string), representing the path to this directory
+  from the root of the ``revision``
+* for ``content``: all those above, plus ``directory`` (a ``sha1_git``)
+
+All keys are optional, but should be provided whenever possible.