diff --git a/docs/moving_bodies.md b/docs/moving_bodies.md
index df78935..dc04213 100644
--- a/docs/moving_bodies.md
+++ b/docs/moving_bodies.md
@@ -212,10 +212,14 @@ refilled cells, `B_m` external-boundary mass flux; collision conserves mass).
 
 Sample point (fixed): post-collision storage after the swap of step n, i.e.
 the same level `G` that M1 reads at step n+1; both ledgers are evaluated
-there. Ledger residual = left side minus every listed term. In a closed
-domain (no inlet/outlet, no forcing) `B = S = 0` and the residual must be
-rounding-level; with open boundaries `B`, `S` are not separately measured, so
-the residual is reported as their estimate and is not a pass criterion.
+there. Ledger residual = left side minus every listed term. Zero mass flux
+does not imply zero momentum exchange: closed walls (bounce-back, free-slip)
+still exchange momentum, so "no inlet/outlet, no forcing" does NOT give
+`B = S = 0`. The rounding-level momentum check therefore uses either a
+momentum-isolated case (periodic in both directions, no forcing) or counts
+the wall impulse explicitly as part of `B`; the mass check may use a closed
+domain. Otherwise `B`, `S` are not separately measured, the residual is
+reported as their estimate and is not a pass criterion.
 Both ledger identities are **proposed, unreviewed**: they are not called
 proven until review and a host replay agree.
 
@@ -242,7 +246,10 @@ above the declared bound.
 - Host motion state is committed only after the upload *and* the following
   synchronization of the step group succeed (section 4); any failure in
   between makes the simulation unusable and no host/device match is reported.
-- Device-side soft diagnostics (refill fallbacks) are counters, not errors.
+- The only soft device diagnostic is `eq_only` (NEQ part omitted, counted).
+  `no_donor > 0` is a hard failure: it is checked after the step group and
+  before the step is committed or any results are returned; on failure the
+  simulation is unusable (section 5).
 
 ## 8. Coupling and units (CANDIDATE)