diff --git a/docs/re/BACKLOG.md b/docs/re/BACKLOG.md
index 5c7c4de..e523cb3 100644
--- a/docs/re/BACKLOG.md
+++ b/docs/re/BACKLOG.md
@@ -6098,3 +6098,50 @@ Corrected in `asteroid-fields.md` and `INDEX.md`.
 
 🟡 Not settled: whether the quaternion is object→world or world→object; the two
 unnamed `_Test` variant objects; what `Placement_*`/`RouteTest_*` are for.
+
+---
+
+## ✅✅ 2026-08-28 — every challenge mission is flown on a story mission's map
+
+Item (a): apply the per-stage join to the other stage-resource fields. **Sharing
+is exactly what the file-name view hides**, and here it is, complete.
+
+Of 338 distinct `(field, value)` pairs across the 22 stage objects, **31 are used
+by more than one stage.** The strongest signal is `CollisionMeshes`:
+
+| stage | `CollisionMeshes` | `EnumerateSubobjective` | `BackGroundID` | `EnumerateWeapon` |
+|---|---|---|---|---|
+| S24 | `CollisionSet_S11.bin` | `SUBObjectiveSettings_S01` | `Anastasis` | `EnumWeapon_Test` |
+| S25 | `CollisionSet_S07.bin` | `SUBObjectiveSettings_S07` | `Hargenteen` | `EnumWeapon_Test` |
+| S26 | `CollisionSet_S03.bin` | `SUBObjectiveSettings_S06` | `Hargenteen` | `EnumWeapon_Test` |
+| S27 | `CollisionSet_S03.bin` | `SUBObjectiveSettings_S03` | `Planet_Lebendorf` | `EnumWeapon_Test` |
+| S28 | `CollisionSet_S14.bin` | `SUBObjectiveSettings_S01` | `Lebendorf` | **`EnumWeapon_EX5.tbl`** |
+| S29 | `CollisionSet_S15.bin` | `SUBObjectiveSettings_S15` | `Earth` | `EnumWeapon_Test` |
+
+**6 of 6 challenge missions reuse a story stage's collision set.** Control:
+**S01–S16 are perfectly diagonal**, `CollisionSet_SNN` ↔
+`SUBObjectiveSettings_SNN`, **16/16**.
+
+🔑 **Map, objectives and sky are borrowed independently** — S24 flies S11's map
+with S01's objectives; S26 flies S03's map with S06's objectives under a
+`Hargenteen` sky belonging to neither; S28 flies S14's map with S01's objectives
+under S01's sky. Three of six mix, so "S24 *is* S11" would be wrong.
+
+🔑 **S28 is the only stage with its own weapon table** — 21 stages use
+`EnumWeapon_Test.tbl` (×132), the tutorials `EnumWeapon_Tutorial.tbl` (×36), and
+S28 alone `EnumWeapon_EX5.tbl` (×6). S28 was already the odd stage (the only
+`rou_f004` flight); this is a second hook on it.
+
+❔ **A datapoint for [[result-screens]]'s open "what does `EX_` mean"**: `EX5` is
+also a **variant suffix on real names** — `UN_f001_TCAF_DeltaSaber_T_EX5` and
+`Weapon_TCAF_Ship_AAGun_EX5`, 24 each. Not yet enough to fix the meaning.
+
+⚠️ **Method note worth keeping**: an *unanchored* `EX[0-9]` search over binary
+payloads returns `EX0`…`EX9` at 100–180 hits each — pure noise from matching
+inside compressed data. Only prefix-anchored names are real. (Sibling of "a
+prefix match is not an exact match".)
+
+Written up in `docs/re/structures/stage-numbering-and-player-craft.md`.
+
+🟡 Not settled: what `EX` stands for; the two unnamed `_Test` variant objects;
+what `Placement_*`/`RouteTest_*` are for.
diff --git a/docs/re/structures/stage-numbering-and-player-craft.md b/docs/re/structures/stage-numbering-and-player-craft.md
index 6914b11..3805aba 100644
--- a/docs/re/structures/stage-numbering-and-player-craft.md
+++ b/docs/re/structures/stage-numbering-and-player-craft.md
@@ -119,3 +119,45 @@ running 0…27.
   `<lang>\GP_HANGAR_ARSENAL_3D.tbl` all resolve, so the method works).
 * Why the numbering skips S17 and parks the tutorials at S18–S23 is a production
   fact this cannot reach.
+
+## ✅ Every challenge mission is flown on a story mission's map (2026-08-28)
+
+Joining the stage records **per stage** rather than per file name — the same move
+that showed S28 borrowing S14's asteroid table ([[asteroid-fields]]):
+
+| stage | `CollisionMeshes` | `EnumerateSubobjective` | `BackGroundID` | `EnumerateWeapon` |
+|---|---|---|---|---|
+| S24 | **`CollisionSet_S11.bin`** | `SUBObjectiveSettings_S01` | `Anastasis` (S11's) | `EnumWeapon_Test` |
+| S25 | **`CollisionSet_S07.bin`** | `SUBObjectiveSettings_S07` | `Hargenteen` (S07's) | `EnumWeapon_Test` |
+| S26 | **`CollisionSet_S03.bin`** | `SUBObjectiveSettings_S06` | `Hargenteen` | `EnumWeapon_Test` |
+| S27 | **`CollisionSet_S03.bin`** | `SUBObjectiveSettings_S03` | `Planet_Lebendorf` (S03's) | `EnumWeapon_Test` |
+| S28 | **`CollisionSet_S14.bin`** | `SUBObjectiveSettings_S01` | `Lebendorf` (S01's) | **`EnumWeapon_EX5.tbl`** |
+| S29 | **`CollisionSet_S15.bin`** | `SUBObjectiveSettings_S15` | `Earth` (S15's) | `EnumWeapon_Test` |
+
+**6 of 6 challenge missions reuse a story stage's collision set**, and the control
+is exact: **S01–S16 are perfectly diagonal**, `CollisionSet_SNN` ↔
+`SUBObjectiveSettings_SNN` for their own `NN`, **16/16**. (The 22 stage objects
+are the 16 story + 6 challenge; the tutorials share tables, as
+[[stage-mission-tables]] records.)
+
+🔑 **Map, objectives and sky are borrowed independently.** S24 flies **S11's map**
+with **S01's objectives**; S26 flies **S03's map** with **S06's objectives** under
+a `Hargenteen` sky that belongs to neither; S28 flies **S14's map** with **S01's
+objectives** under **S01's sky**. Three of the six mix sources, so "S24 *is* S11"
+would be wrong — the re-dressing is per field.
+
+🔑 **S28 is the only stage on the disc with its own weapon table.** All 21 others
+use `EnumWeapon_Test.tbl` (×132) and the tutorials `EnumWeapon_Tutorial.tbl`
+(×36); S28 alone declares **`EnumWeapon_EX5.tbl`** (×6, one per language pack).
+A field with exactly one distinctive value is an identification hook, and S28 was
+already the odd stage — the only one flying `rou_f004`.
+
+❔ **A datapoint for the open "what does `EX_` mean"** ([[result-screens]]): `EX5`
+is also a **variant suffix on real unit and weapon names** —
+`UN_f001_TCAF_DeltaSaber_T_EX5` and `Weapon_TCAF_Ship_AAGun_EX5`, 24 occurrences
+each (4 per pack). So `EX5` names a *variant*, and one stage's weapon list is
+built for it. Not yet enough to say what the letters stand for.
+
+⚠️ Method note: an **unanchored** `EX[0-9]` search over the binary payloads
+returns `EX0`…`EX9` at 100–180 hits each — pure noise from matching inside
+compressed data. Only the prefix-anchored names above are real.