diff --git a/docs/Historian.md b/docs/Historian.md
index 71072c65..74f01acf 100644
--- a/docs/Historian.md
+++ b/docs/Historian.md
@@ -163,7 +163,12 @@ data.
   read and there is no "full page ⇒ maybe more" signal to page on. Returning the full result with
   no continuation point is spec-conformant.
 - **No modified-value history** (`HistoryReadModified`). Requests for modified values return
-  `BadHistoryOperationUnsupported`. This is a follow-up.
+  `BadHistoryOperationUnsupported`. This is **infra-gated, not a server-code gap**: the AVEVA
+  Wonderware historian backend (`IHistorianDataSource`, the TCP sidecar client) exposes only a
+  current-value read path — there is no modified/edited-history surface to source the data from. The
+  server-side override is in place (it cleanly rejects modified reads per node) and `IsReadModified`
+  is honoured; serving real modified-value history is unblocked only once the historian client/sidecar
+  grows a modified-read RPC. Until then, rejecting is the correct, spec-conformant behaviour.
 
 ### Redundancy and authorization