diff --git a/mypy-files.txt b/mypy-files.txt
index 5dcd3044207a..1b56b5e8ea18 100644
--- a/mypy-files.txt
+++ b/mypy-files.txt
@@ -1,6 +1,7 @@
 nova/compute/manager.py
 nova/crypto.py
 nova/limit/local.py
+nova/limit/placement.py
 nova/network/neutron.py
 nova/pci
 nova/privsep/path.py
diff --git a/nova/limit/local.py b/nova/limit/local.py
index 38e7db05b7df..f4e02c802073 100644
--- a/nova/limit/local.py
+++ b/nova/limit/local.py
@@ -20,7 +20,6 @@ from oslo_limit import limit
 from oslo_log import log as logging
 
 import nova.conf
-from nova import context as nova_context
 from nova import exception
 from nova.limit import utils as nova_limit_utils
 from nova import objects
@@ -80,7 +79,9 @@ LEGACY_LIMITS = {
 }
 
 
-def get_in_use(context, project_id):
+def get_in_use(
+    context: 'nova.context.RequestContext', project_id: str
+) -> ty.Dict[str, int]:
     """Returns in use counts for each resource, for given project.
 
     This sounds simple but many resources can't be counted per project,
@@ -144,7 +145,7 @@ def enforce_api_limit(entity_type: str, count: int) -> None:
 
 
 def enforce_db_limit(
-    context: nova_context.RequestContext,
+    context: 'nova.context.RequestContext',
     entity_type: str,
     entity_scope: ty.Any,
     delta: int
@@ -192,7 +193,9 @@ def enforce_db_limit(
         raise EXCEPTIONS.get(entity_type, exception.OverQuota)(str(e))
 
 
-def _convert_keys_to_legacy_name(new_dict):
+def _convert_keys_to_legacy_name(
+    new_dict: ty.Dict[str, int]
+) -> ty.Dict[str, int]:
     legacy = {}
     for new_name, old_name in LEGACY_LIMITS.items():
         # defensive incase oslo or keystone doesn't give us an answer
@@ -200,7 +203,7 @@ def _convert_keys_to_legacy_name(new_dict):
     return legacy
 
 
-def get_legacy_default_limits():
+def get_legacy_default_limits() -> ty.Dict[str, int]:
     # TODO(johngarbutt): need oslo.limit API for this, it should do caching
     enforcer = limit.Enforcer(lambda: None)
     new_limits = enforcer.get_registered_limits(LEGACY_LIMITS.keys())
diff --git a/nova/limit/placement.py b/nova/limit/placement.py
index 95beffaff068..497986c4ab87 100644
--- a/nova/limit/placement.py
+++ b/nova/limit/placement.py
@@ -12,6 +12,7 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import typing as ty
 
 import os_resource_classes as orc
 from oslo_limit import exception as limit_exceptions
@@ -39,17 +40,23 @@ LEGACY_LIMITS = {
 }
 
 
-def _get_placement_usages(context, project_id):
+def _get_placement_usages(
+    context: 'nova.context.RequestContext', project_id: str
+) -> ty.Dict[str, int]:
     global PLACEMENT_CLIENT
     if not PLACEMENT_CLIENT:
         PLACEMENT_CLIENT = report.SchedulerReportClient()
     return PLACEMENT_CLIENT.get_usages_counts_for_limits(context, project_id)
 
 
-def _get_usage(context, project_id, resource_names):
+def _get_usage(
+    context: 'nova.context.RequestContext',
+    project_id: str,
+    resource_names: ty.List[str],
+) -> ty.Dict[str, int]:
     """Called by oslo_limit's enforcer"""
     if not limit_utils.use_unified_limits():
-        raise NotImplementedError("unified limits is disabled")
+        raise NotImplementedError("Unified limits support is disabled")
 
     count_servers = False
     resource_classes = []
@@ -113,7 +120,9 @@ def _get_usage(context, project_id, resource_names):
     return resource_counts
 
 
-def _get_deltas_by_flavor(flavor, is_bfv, count):
+def _get_deltas_by_flavor(
+    flavor: 'objects.Flavor', is_bfv: bool, count: int
+) -> ty.Dict[str, int]:
     if flavor is None:
         raise ValueError("flavor")
     if count < 0:
@@ -132,7 +141,9 @@ def _get_deltas_by_flavor(flavor, is_bfv, count):
     return deltas
 
 
-def _get_enforcer(context, project_id):
+def _get_enforcer(
+    context: 'nova.context.RequestContext', project_id: str
+) -> limit.Enforcer:
     # NOTE(johngarbutt) should we move context arg into oslo.limit?
     def callback(project_id, resource_names):
         return _get_usage(context, project_id, resource_names)
@@ -140,8 +151,15 @@ def _get_enforcer(context, project_id):
     return limit.Enforcer(callback)
 
 
-def enforce_num_instances_and_flavor(context, project_id, flavor, is_bfvm,
-                                     min_count, max_count, enforcer=None):
+def enforce_num_instances_and_flavor(
+    context: 'nova.context.RequestContext',
+    project_id: str,
+    flavor: 'objects.Flavor',
+    is_bfvm: bool,
+    min_count: int,
+    max_count: int,
+    enforcer: ty.Optional[limit.Enforcer] = None
+) -> int:
     """Return max instances possible, else raise TooManyInstances exception."""
     if not limit_utils.use_unified_limits():
         return max_count
diff --git a/nova/quota.py b/nova/quota.py
index 9390e3e2f2e2..b9dd76301276 100644
--- a/nova/quota.py
+++ b/nova/quota.py
@@ -1243,6 +1243,26 @@ def _server_group_count_members_by_user_legacy(context, group, user_id):
 
 
 def is_qfd_populated(context):
+    """Check if user_id and queued_for_delete fields are populated.
+
+    This method is related to counting quota usage from placement. It is not
+    yet possible to count instances from placement, so in the meantime we can
+    use instance mappings for counting. This method is used to determine
+    whether the user_id and queued_for_delete columns are populated in the API
+    database's instance_mappings table. Instance mapping records are not
+    deleted from the database until the database is archived, so
+    queued_for_delete tells us whether or not we should count them for instance
+    quota usage. The user_id field enables us to scope instance quota usage to
+    a user (legacy quota).
+
+    Scoping instance quota to a user is only possible
+    when counting quota usage from placement is configured and unified limits
+    is not configured. When unified limits is configured, quotas are scoped
+    only to projects.
+
+    In the future when it is possible to count instance usage from placement,
+    this method will no longer be needed.
+    """
     global UID_QFD_POPULATED_CACHE_ALL
     if not UID_QFD_POPULATED_CACHE_ALL:
         LOG.debug('Checking whether user_id and queued_for_delete are '
diff --git a/nova/tests/unit/limit/test_placement.py b/nova/tests/unit/limit/test_placement.py
index 790ee3bdbe1b..cfb6ce36b478 100644
--- a/nova/tests/unit/limit/test_placement.py
+++ b/nova/tests/unit/limit/test_placement.py
@@ -79,7 +79,7 @@ class TestGetUsage(test.NoDBTestCase):
         self.flags(driver="nova.quota.NoopQuotaDriver", group="quota")
         e = self.assertRaises(NotImplementedError, placement_limits._get_usage,
                               self.context, uuids.project, [])
-        self.assertEqual("unified limits is disabled", str(e))
+        self.assertEqual("Unified limits support is disabled", str(e))
 
     @mock.patch.object(quota, "is_qfd_populated")
     @mock.patch.object(objects.InstanceMappingList, "get_counts")