Index: docs/EXPORT-FORMAT.md
===================================================================
--- docs/EXPORT-FORMAT.md	(revision 621126bfda9f0449bb84e2e4e25257855a80b5c7)
+++ docs/EXPORT-FORMAT.md	(revision 712b1615e6ae1b194be929712df713bcd9082041)
@@ -102,5 +102,5 @@
 | `updated` | `posts.updated_at` | omitted when equal to `published` |
 | `url` | `<origin>/<slug>` | the human permalink |
-| `attachment` | cover, inline images, `c2s_attachments` | see [Media](#media) |
+| `attachment` | cover, inline `<img>`, `c2s_attachments` **and their `poster`**, hosted audio tracks | see [Media](#media) |
 | `tag` | `posts.tags`, mentions, custom emoji | `Hashtag`, `Mention`, `toot:Emoji` |
 | `oneOf` / `anyOf` / `endTime` | `posts.poll_json` | a poll exports as a `Question` |
@@ -258,6 +258,9 @@
 ## Reproducibility
 
-Two exports of unchanged content must be **byte-identical**. That is what makes
-an archive verifiable and a diff meaningful.
+Two exports of unchanged content must be **byte-identical except for
+`manifest.exportedAt`**, which is by definition the moment of export. Everything
+else — every post file, every media file, and the `files` checksum map — must
+match. That is what makes an archive verifiable and a diff meaningful: if the
+checksum map is equal, nothing changed.
 
 - JSON: keys sorted, two-space indent, LF, trailing newline.