diff --git a/oups/aggstream/aggstream.py b/oups/aggstream/aggstream.py
index c76e43a..a55fd6c 100644
--- a/oups/aggstream/aggstream.py
+++ b/oups/aggstream/aggstream.py
@@ -398,6 +398,9 @@ def _iter_data(
     """
     Iterate provided seed, applying sequentially (optionally) filters.
 
+    Seed has to be monotonic increasing on 'ordered_on' column. If not, it is
+    ordered.
+
     Parameters
     ----------
     seed : Iterable[pDataFrame]