diff --git a/.project/concepts/app-contract/contract_polyfill-surface.md b/.project/concepts/app-contract/contract_polyfill-surface.md
index efdc269..37df9b7 100644
--- a/.project/concepts/app-contract/contract_polyfill-surface.md
+++ b/.project/concepts/app-contract/contract_polyfill-surface.md
@@ -122,7 +122,7 @@ It resolves **only once that work has actually completed**: if what was shared w
 
 **The session is the package's, not yours.** You never build one, and no call takes one. Call this package's `init` (not the one you passed to `configure`): it captures the session the SDK delivers to `init`'s callback and keeps it, then calls your callback with that same event untouched — so an application that wants the `session_id` for the `docs` primitives reads it there, and one that does not may pass no callback at all. Identity normalisation is the package's too: `@Alice`, `alice ` and `ALICE` are one person.
 
-`createEntityDoc` throws if the document cannot be recorded in its store.
+Where a call must first find out whether something already exists — a document's record in its store, a user's inbox — it throws when it could not find out, instead of proceeding as though the answer were "nothing". So `createEntityDoc` throws if the document cannot be recorded in its store, and resolving an inbox throws rather than handing back a second one. **A rejection means "unknown", never "absent"** — retry it or surface it, but do not read it as an empty result.
 
 ## Non-guarantees