diff --git a/docs/formats/core-data-regions.md b/docs/formats/core-data-regions.md
index 0a5810d..1726b44 100644
--- a/docs/formats/core-data-regions.md
+++ b/docs/formats/core-data-regions.md
@@ -1747,8 +1747,32 @@ The remaining ~6% are **not 12-bit fixed at all**:
 
 **What this establishes:** `i16[3]` is a per-vertex packed attribute whose value
 *selects how the texcoord pair is stored*, rather than being an independent field
-sitting beside the texcoords. Values 1-255 are the ordinary case; 0 is special;
-256 and above, and negatives, are a different encoding entirely.
+sitting beside the texcoords.
+
+**Refined: it is the HIGH BYTE that selects.** Grouped by the high byte across the
+same 95,612 vertices on levels 0-2:
+
+    i16[3] high byte        n        % / 16      % / 256
+    0x00                89,341        92.0%        91.8%
+    0x03                 5,344         0.0%         0.0%
+    0x04                    71         0.0%         0.0%
+    0x40                    58         5.2%         0.0%
+    0x02                    47         2.1%         2.1%
+    0x19, 0x05, 0x20, ...  the remaining 73
+
+A high byte of **zero** is the ordinary case -- 89,341 of 95,612, or **93.4%** --
+and it scores 92%. **Every** other high byte scores at or near zero. The split is
+on the byte, not on the value: `i16[3]` values 1 and 771 differ only in the high
+byte and behave completely differently.
+
+Within the all-zero case there is a third distribution: `i16[3] == 0` exactly
+(5,851 vertices) scores 62% at 16 and 61% at 256, against 92% for `1..255`.
+
+**So three cases, selected by the high byte and then the low:**
+
+    high 0x00, low 0x00    5,851    a third distribution, 62% at 1/16
+    high 0x00, low 1..255 83,490   the ordinary case, 92% at 1/4096
+    high non-zero         6,271    0%, an encoding this reading does not see
 
 **Not established:** what any of those three cases means. Whether `i16[3]` is a
 texture index, a normal index, a flag or a combination is still open -- but it is
diff --git a/docs/roadmap.md b/docs/roadmap.md
index 0e63719..2ac286b 100644
--- a/docs/roadmap.md
+++ b/docs/roadmap.md
@@ -293,14 +293,14 @@ Phase 1 hands over the extents; this phase works out what each one contains.
       against 1.1% for `(0,1)` and 4.8% for `(2,3)`. The `(6,7)` pair scores
       *higher* at 76.6% and is **not** a texcoord -- it is the near-constant
       `4096, 0` pair, which is trivially two multiples of 4,096.
-- [x] `i16[3]` **selects the texcoord encoding**, which explains the residual.
-      Grouped across 95,612 vertices on levels 0-2: `1..63` and `64..255` score
-      92.7% and 92.3% 12-bit fixed, `== 0` scores 29.2% at 4,096 but **60.7% at
-      256** -- a third distribution -- and everything at 256 or above, or
-      negative, scores **0%** at both. The 87.3% of vertices in `1..255`
-      contribute exactly the 82.5% measured overall, so the residual is fully
-      accounted for. It is a field whose *role* is known even though its meaning
-      is not.
+- [x] `i16[3]` **selects the texcoord encoding**, and it is specifically the
+      **high byte**. Grouped across 95,612 vertices on levels 0-2: high byte `0x00`
+      covers 89,341 vertices (93.4%) at 92% 12-bit fixed, while **every** other
+      high byte scores 0%. The split is on the byte, not the value -- `i16[3]` of 1
+      and of 771 differ only in the high byte and behave completely differently.
+      Within the all-zero case, `i16[3] == 0` exactly (5,851) is a third
+      distribution at 62%. Three cases: `0x00/0x00`, `0x00/1..255`, and non-zero
+      high byte.
 - [ ] The remaining tie vertex **fields are not named.** `i16[0..3]` span the full
       `i16` range with 10,803-13,421 distinct values each. `i16[0..2]` are most
       likely position, but that is not established.