diff --git a/docs/pipeline-rationale.md b/docs/pipeline-rationale.md
index 61bc3c4..9454e5a 100644
--- a/docs/pipeline-rationale.md
+++ b/docs/pipeline-rationale.md
@@ -19,6 +19,16 @@ not yet reviewed and not yet a citation target, so its being gitignored and read
 ingest session does not weaken anything - the boundary is the promotion into `raw/` itself, not
 the moment a human happened to drop a file somewhere.
 
+Once a document can arrive from *outside* - the MCP server's optional `submit` tool - "a human
+happened to drop a file somewhere" stops describing how everything reaches `incoming/`, so the
+near side of the boundary gets a stage of its own rather than a second meaning. `mcp-upload/`
+holds what nobody has looked at yet; `incoming/` holds what someone has. Two arrows, two
+different things being granted: `upload accept` grants *trust* (a human read the material and
+took responsibility for it), `raw accept` grants *immutability* (it becomes a citation target
+and stops being editable). Collapsing them would have meant one of the two lying - either an
+unreviewed stranger's file sitting in the same directory a human's own drop does, or the
+promotion into `raw/` quietly doubling as the review step it cannot perform.
+
 ## Why extraction happens once, through a schema
 
 [types/type-spec.md](../types/type-spec.md) is what stands between a raw file and a `kb/` page: