diff --git a/docs/Historian.md b/docs/Historian.md
index ae1568b3..5b7fb1c2 100644
--- a/docs/Historian.md
+++ b/docs/Historian.md
@@ -273,6 +273,22 @@ for this node), which is the signal to check that `ServerHistorian:Enabled=true`
 > so a HistoryRead on it runs to the full `CallTimeout` (~30 s) before erroring. Auto-provisioning on deploy
 > keeps freshly-historized tags from hitting that slow path. See the `CallTimeout` row above.
 
+### Deferred types and the UInt16 fallback (#600 item 4)
+
+Historization is **numeric-analog only** in v1. `HistorianTypeMapper` classifies exactly nine types as
+historizable (`Boolean`, `Int16/32/64`, `UInt16/32/64`, `Float32`, `Float64`):
+
+- **Deferred: `String`, `DateTime`, `Reference`.** The historian's SQL analog write path cannot carry them.
+  Provisioning skips them (counted in the `skipped=` tally, via `IsHistorizable`), the mapper throws
+  `NotSupportedException` for them, and `ContinuousHistorizationRecorder` drops their values and counts them
+  in `DroppedNonNumeric`. A tag authored `isHistorized` with one of these types therefore silently records
+  nothing; HistoryRead on it returns whatever the historian holds (typically `GoodNoData`).
+- **Fallback: `UInt16 -> Uint4`.** The historian's `UInt2` write path is unproven upstream, so UInt16 tags
+  are provisioned as the wider `UInt4`. Values are unaffected; only the historian-side storage type differs.
+  (`Boolean -> Int2` is the other fallback, below.)
+
+Lifting either needs gateway-side support first; neither is scheduled.
+
 ### Boolean historization maps to Int2 (issue #441)
 
 `EnsureTags` is contract-documented (HistorianGateway 0.2.0+) as an **upsert** — *"creates or updates