diff --git a/docs/proposals/module-boundaries.en.md b/docs/proposals/module-boundaries.en.md
index 98be738..c9fea5a 100644
--- a/docs/proposals/module-boundaries.en.md
+++ b/docs/proposals/module-boundaries.en.md
@@ -66,6 +66,26 @@ ETAF also does not define a public `spacer` type. A Box with no children is an e
 Box; Ebox may use an empty-node representation or private helper, but users do not
 need another visual category.
 
+### 1.3 Should Box expose an outer participation mode?
+
+The target does not expose `inline`/`block` as a second public ETAF layout axis yet:
+
+- the parent Box already decides child placement through `column`, `row`, `flex`, or
+  `grid`;
+- `Text` owns inline runs, wrapping, and text measurement;
+- most ETAF Boxes are layout items whose position is owned by the parent layout;
+- adding `:outer 'inline` would also require inline boxes, anonymous line boxes, mixed
+  block/inline flow, and corresponding incremental proofs.
+
+The target public API therefore exposes `Box :layout ...` and inline content through
+`Text`. If a real inline badge, inline-flex, or mixed inline/block requirement appears
+later, add an orthogonal `:outer` property. It must remain independent from `:layout`
+because CSS `inline-flex` means outer inline plus inner flex. Do not turn the two axes
+into one growing enum or replace them with a Component.
+
+Ebox may keep an outer/inner display pair internally; that backend choice does not make
+both axes ETAF public API.
+
 ## 2. Module responsibilities
 
 ### ETAF Core
diff --git a/docs/proposals/module-boundaries.zh.md b/docs/proposals/module-boundaries.zh.md
index 6abec54..9eb94c3 100644
--- a/docs/proposals/module-boundaries.zh.md
+++ b/docs/proposals/module-boundaries.zh.md
@@ -81,7 +81,34 @@ column | row | flex | grid
 文本应用不需要把完整 CSS `display` 矩阵暴露给用户；未来若 Ebox 确实需要，
 应增加正交属性，而不是增加 `inline` Component。
 
-### 1.3 子节点与后端 content
+### 1.3 是否需要 Box 的外部参与方式
+
+当前目标不把 `inline`/`block` 暴露为 ETAF 的第二个公共布局轴，原因是：
+
+- ETAF 的父级 `Box` 已经通过 `column`、`row`、`flex` 或 `grid` 明确决定子节点
+  如何参与布局；
+- `Text` 自己负责 inline runs、换行和文字测量；
+- 大多数 ETAF Box 都是布局项，父级布局拥有它们的位置，不需要再由子节点声明
+  一套 CSS 外部参与规则；
+- 引入 `:outer 'inline` 会同时要求 inline box、匿名 line box、混合 block/inline
+  流和对应的增量证明，超出当前文本应用的最小模型。
+
+因此目标公共 API 只有：
+
+```text
+Box :layout ...
+Text inline content
+```
+
+如果未来出现真实的 inline badge、inline-flex 或混合 inline/block 需求，再增加
+正交的 `:outer` 属性，例如 `:outer 'inline` 或 `:outer 'block`。它必须独立于
+`:layout`，因为 CSS 的 `inline-flex` 本身就是“外部 inline、内部 flex”；不能把
+它们拼成一个越来越大的 `:layout` 枚举，也不能用 Component 代替。
+
+当前 Ebox 内部保留 outer/inner display pair 是后端实现选择，不代表 ETAF 必须
+暴露这两个轴。
+
+### 1.4 子节点与后端 content
 
 目标 ETAF 语法只保留一个内容模型：子节点。