diff --git a/docs/topics/throttling.rst b/docs/topics/throttling.rst
index 99a5fe0d7..2de8cea8d 100644
--- a/docs/topics/throttling.rst
+++ b/docs/topics/throttling.rst
@@ -1044,6 +1044,14 @@ Additional settings
     <throttling-scopes>` that are neither domains nor IPs, e.g. custom scopes
     added by a :ref:`custom throttling manager <custom-throttling-scopes>`.
 
+    A scope counts as a domain only if it looks like a hostname with at least
+    one dot, so single-label hosts such as ``localhost`` (or intranet host
+    names) fall in this category and default to
+    :setting:`THROTTLING_SCOPE_CONCURRENCY` rather than
+    :setting:`CONCURRENT_REQUESTS_PER_DOMAIN`. Raise it, or give the host an
+    explicit ``concurrency`` in :setting:`THROTTLING_SCOPES`, if you need more
+    concurrency against such hosts (e.g. a local development server).
+
     Domain and IP scopes use :setting:`CONCURRENT_REQUESTS_PER_DOMAIN` and
     :setting:`CONCURRENT_REQUESTS_PER_IP` instead. A scope ``concurrency`` set
     in :setting:`THROTTLING_SCOPES` overrides this.
@@ -1101,3 +1109,4 @@ API
 
 .. autofunction:: scrapy.throttling.scope_cache
 .. autofunction:: scrapy.throttling.add_scope
+.. autofunction:: scrapy.throttling.iter_scopes
diff --git a/scrapy/throttling.py b/scrapy/throttling.py
index eec7c9ddd..356571a05 100644
--- a/scrapy/throttling.py
+++ b/scrapy/throttling.py
@@ -98,6 +98,14 @@ RequestScopes = None | ScopeID | Iterable[ScopeID] | dict[ScopeID, float | None]
 
 
 def iter_scopes(scopes: RequestScopes) -> Iterable[ScopeID]:
+    """Iterate over the scope IDs of *scopes*, whatever its form.
+
+    :class:`~ThrottlingManagerProtocol.get_scopes` (and
+    :meth:`~ThrottlingManagerProtocol.get_resolved_scopes`) may return a single
+    scope ID, an iterable of them, a ``{scope_id: quota}`` mapping, or ``None``;
+    this helper normalizes any of those into an iterable of scope IDs, e.g. to
+    react to a request's scopes in a custom middleware.
+    """
     if scopes is None:
         return ()
     if isinstance(scopes, str):