diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index b30b8177..2c762a9e 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -3241,7 +3241,14 @@ def _synthesise_17_0_sap_windows(schema: RdSapSchema17_0) -> List[SapWindow]:
     core (968/1000 band-1 with no measured band-1 windows; only 10 rich certs).
     17.0 glazed_type codes (observed 1-4, 7) are a subset of 20.0.0's verified
     1-8 space: route integer codes through the verified cascade; the "ND" string
-    falls back to the DG-modal default. Own seam so 17.0 can diverge."""
+    falls back to the DG-modal default. Own seam so 17.0 can diverge.
+
+    Transfer note (ADR-0028 validate-don't-re-fit): 17.0's Validation Cohort
+    splits two ways — band-4 (n=6) observes 0.170 vs predicted 0.223 (under),
+    band-5 (n=4) observes 0.150 vs predicted 0.092 (over). Both cohorts are tiny
+    and at the glazed extremes; band-1 (96.8%) is unfittable, so inherit stands.
+    Revisit only if synthesis fidelity (scoring) needs it. See
+    `scripts/eon/profile_corpus.py`."""
     mgt = schema.multiple_glazing_type
     glazing_type = (
         _api_cascade_glazing_type(mgt)