diff --git a/docs/plans/2026-10-09-pluggable-historian-design.md b/docs/plans/2026-10-09-pluggable-historian-design.md
index c22d853c..758c0522 100644
--- a/docs/plans/2026-10-09-pluggable-historian-design.md
+++ b/docs/plans/2026-10-09-pluggable-historian-design.md
@@ -213,7 +213,10 @@ next call (not latched).
   value + status. Otherwise Double/Int64 linearly interpolated (status `Good` with the historian
   *Interpolated* info bits), Boolean/String/DateTime stepped from the previous
   sample (status keeps the previous sample's code + Interpolated bit); no previous sample ⇒ `BadNoData`.
-  If either neighbour is Bad, the result is `UncertainDataSubNormal`.
+  If the **previous** neighbour is Bad, the requested time lies inside an outage: the result is a null value
+  carrying that neighbour's Bad code (never better than its source, per the quality contract; corrected after the
+  T10 review). If the previous neighbour is usable and the **next** is Bad, the previous value is held (stepped,
+  not interpolated into the outage) with `UncertainDataSubNormal`.
 - **Events:** `TOP(@maxEvents)` from `{s}.Event` filtered by `SourceName = @source` (null ⇒ all) and
   `EventTimeUtc` range, ordered ascending. `HistoricalEvent(EventId, SourceName, EventTimeUtc,
   ReceivedTimeUtc, Message, (ushort)Severity)`.