--- a/src/CelerisLab/lbm/refinement_stepper.py
+++ b/src/CelerisLab/lbm/refinement_stepper.py
@@ -32,6 +32,7 @@
 
     ``coarse_boundary`` is ``periodic`` (default) or ``slot_reservoir``:
     a bottom on-site slot and top fixed-density reservoir from ``cfg``.
+    Periodic mode does not apply the wall/slot boundary fields in ``cfg``.
     Physical walls remain coarse; fine halo donors must be ordinary fluid.
 
     ``initialize(coarse_g, fine_g)`` accepts physical Q-major (9,ny,nx) arrays