--- a/src/CelerisLab/lbm/refinement_stepper.py
+++ b/src/CelerisLab/lbm/refinement_stepper.py
@@ -39,6 +39,7 @@
     velocities are in coarse lattice units (dx=dt=1). Independent local copies
     are projected at fine cell centers. Their complete stencil must be active
     and disjoint from the actual two-hop transfer footprint.
+    These use the edge/cell-center frame (coarse centers I+.5), whereas ordinary Simulation objects use the integer-node frame.
 
     ``initialize(coarse_g, fine_g)`` accepts physical Q-major (9,ny,nx) arrays
     already in postcollision phase. It does not collide or build channel flags.