diff --git a/swh/loader/package/functional/loader.py b/swh/loader/package/functional/loader.py
--- a/swh/loader/package/functional/loader.py
+++ b/swh/loader/package/functional/loader.py
@@ -6,7 +6,9 @@
 import json
 import requests
 
-from typing import Dict, Optional, Any
+from typing import Dict, Optional, Any, Mapping
+
+from swh.model import hashutil
 
 from swh.model.model import (
     Sha1Git, Revision, RevisionType
@@ -36,8 +38,13 @@
 
     def __init__(self, url):
         super().__init__(url=url)
-        self.sources = retrieve_sources(url)['sources']
+        s = retrieve_sources(url)
+        self.sources = s['sources']
         self.provider_url = url
+        # The revision used to create the sources.json file. For Nix,
+        # this revision belongs to the github.com/nixos/nixpkgs
+        # repository
+        self.revision = s['revision']
 
     # Note: this could be renamed get_artifacts in the PackageLoader
     # base class.
@@ -63,6 +70,29 @@
                 return rev_id
         return None
 
+    def extra_branches(self) -> Dict[bytes, Mapping[str, Any]]:
+        """We add a branch to the snapshot called 'evaluation' pointing to the
+        revision used to generate the sources.json file. This revision
+        is specified in the sources.json file itself. For the nixpkgs
+        origin, this revision is coming from the
+        github.com/nixos/nixpkgs repository.
+
+        Note this repository is not loaded explicitly. So, this
+        pointer can target a nonexistent revision for a time. However,
+        the github and gnu loaders are supposed to load this revision
+        and should create the revision pointed by this branch.
+
+        This branch can be used to identify the snapshot associated to
+        a Nix/Guix evaluation.
+
+        """
+        return {
+            b'evaluation': {
+                'target_type': 'revision',
+                'target': hashutil.hash_to_bytes(self.revision)
+            }
+        }
+
     def build_revision(self, a_metadata: Dict, uncompressed_path: str,
                        directory: Sha1Git) -> Optional[Revision]:
         return Revision(
diff --git a/swh/loader/package/functional/tests/data/https_nix-community.github.io/nixpkgs-swh_sources.json b/swh/loader/package/functional/tests/data/https_nix-community.github.io/nixpkgs-swh_sources.json
--- a/swh/loader/package/functional/tests/data/https_nix-community.github.io/nixpkgs-swh_sources.json
+++ b/swh/loader/package/functional/tests/data/https_nix-community.github.io/nixpkgs-swh_sources.json
@@ -9,5 +9,6 @@
       "url": [ "https://example.com/file.txt" ]
     }
  ],
-  "version": 1
+    "version": 1,
+    "revision": "cc4e04c26672dd74e5fd0fecb78b435fb55368f7"
 }
diff --git a/swh/loader/package/functional/tests/data/https_nix-community.github.io/nixpkgs-swh_sources.json_visit1 b/swh/loader/package/functional/tests/data/https_nix-community.github.io/nixpkgs-swh_sources.json_visit1
--- a/swh/loader/package/functional/tests/data/https_nix-community.github.io/nixpkgs-swh_sources.json_visit1
+++ b/swh/loader/package/functional/tests/data/https_nix-community.github.io/nixpkgs-swh_sources.json_visit1
@@ -13,5 +13,6 @@
       "url": [ "https://example.com/file.txt" ]
     }
  ],
-  "version": 1
+  "version": 1,
+  "revision": "602140776b2ce6c9159bcf52ada73a297c063d5e"
 }
diff --git a/swh/loader/package/functional/tests/test_functional.py b/swh/loader/package/functional/tests/test_functional.py
--- a/swh/loader/package/functional/tests/test_functional.py
+++ b/swh/loader/package/functional/tests/test_functional.py
@@ -90,12 +90,16 @@
 
     loader = FunctionalLoader(sources_url)
     loader.load()
-    expected_snapshot_id = '2c7f01ef3115f7999a013979fa27bfa12dcb63eb'
+    expected_snapshot_id = '0c5881c74283793ebe9a09a105a9381e41380383'
     assert load_status == {
         'status': 'eventful',
         'snapshot_id': expected_snapshot_id
     }
     expected_branches = {
+        'evaluation': {
+            'target': 'cc4e04c26672dd74e5fd0fecb78b435fb55368f7',
+            'target_type': 'revision'
+        },
         'https://github.com/owner-1/repository-1/revision-1.tgz': {
             'target': '488ad4e7b8e2511258725063cf43a2b897c503b4',
             'target_type': 'revision'
@@ -128,13 +132,17 @@
     """
     loader = FunctionalLoader(sources_url)
     load_status = loader.load()
-    expected_snapshot_id = '2c7f01ef3115f7999a013979fa27bfa12dcb63eb'
+    expected_snapshot_id = '0c5881c74283793ebe9a09a105a9381e41380383'
     assert load_status == {
         'status': 'eventful',
         'snapshot_id': expected_snapshot_id
     }
 
     expected_branches = {
+        'evaluation': {
+            'target': 'cc4e04c26672dd74e5fd0fecb78b435fb55368f7',
+            'target_type': 'revision'
+        },
         'https://github.com/owner-1/repository-1/revision-1.tgz': {
             'target': '488ad4e7b8e2511258725063cf43a2b897c503b4',
             'target_type': 'revision'
@@ -162,7 +170,7 @@
 
     loader = FunctionalLoader(sources_url)
     load_status = loader.load()
-    expected_snapshot_id = '9c4fbfd991b35c7de876cd66bcda2967a8f476ac'
+    expected_snapshot_id = 'b0bfa75cbd0cc90aac3b9e95fb0f59c731176d97'
     assert load_status == {
         'status': 'eventful',
         'snapshot_id': expected_snapshot_id
@@ -172,6 +180,10 @@
     # second time an url, because of the requests_mock_datadir_visits
     # fixture, the file has to end with `_visit1`.
     expected_branches = {
+        'evaluation': {
+            'target': '602140776b2ce6c9159bcf52ada73a297c063d5e',
+            'target_type': 'revision'
+        },
         'https://github.com/owner-1/repository-1/revision-1.tgz': {
             'target': '488ad4e7b8e2511258725063cf43a2b897c503b4',
             'target_type': 'revision'
@@ -214,3 +226,27 @@
     assert loader.resolve_revision_from(known_artifacts, metadata) == 'id1'
     metadata = {'url': 'url3'}
     assert loader.resolve_revision_from(known_artifacts, metadata) == None # noqa
+
+
+def test_evaluation_branch(swh_config, requests_mock_datadir):
+    loader = FunctionalLoader(sources_url)
+    res = loader.load()
+    assert res['status'] == 'eventful'
+
+    expected_branches = {
+        'https://github.com/owner-1/repository-1/revision-1.tgz': {
+            'target': '488ad4e7b8e2511258725063cf43a2b897c503b4',
+            'target_type': 'revision',
+        },
+        'evaluation': {
+            'target': 'cc4e04c26672dd74e5fd0fecb78b435fb55368f7',
+            'target_type': 'revision',
+        },
+    }
+
+    expected_snapshot = {
+        'id': '0c5881c74283793ebe9a09a105a9381e41380383',
+        'branches': expected_branches,
+    }
+
+    check_snapshot(expected_snapshot, storage=loader.storage)
diff --git a/swh/loader/package/loader.py b/swh/loader/package/loader.py
--- a/swh/loader/package/loader.py
+++ b/swh/loader/package/loader.py
@@ -210,6 +210,13 @@
             uncompress(a_path, dest=uncompressed_path)
         return uncompressed_path
 
+    def extra_branches(self) -> Dict[bytes, Mapping[str, Any]]:
+        """Return an extra dict of branches that are used to update the set of
+        branches.
+
+        """
+        return {}
+
     def load(self) -> Dict:
         """Load for a specific origin the associated contents.
 
@@ -319,6 +326,13 @@
                         'target': target,
                     }
 
+            for name, target in self.extra_branches().items():
+                if name in branches:
+                    logger.error("Extra branch '%s' has been ignored",
+                                 name)
+                else:
+                    branches[name] = target
+
             snapshot_data = {
                 'branches': branches
             }