diff --git a/src/jdk.compiler/share/classes/com/sun/tools/javac/resources/ct.properties b/src/jdk.compiler/share/classes/com/sun/tools/javac/resources/ct.properties
index ba42e8e067c..a8fdd923e2c 100644
--- a/src/jdk.compiler/share/classes/com/sun/tools/javac/resources/ct.properties
+++ b/src/jdk.compiler/share/classes/com/sun/tools/javac/resources/ct.properties
@@ -1,5 +1,5 @@
 #
-# Copyright (c) 2016, 2019, Oracle and/or its affiliates. All rights reserved.
+# Copyright (c) 2016, 2025, Oracle and/or its affiliates. All rights reserved.
 # DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 #
 # This code is free software; you can redistribute it and/or modify it
@@ -23,6 +23,15 @@
 # questions.
 #
 
+# This file was originally used to produce the `proprietary` warning when
+# accessing non-APIs, and to hide some non-APIs in JDK <= 8. This has been
+# superseded by the module system and `--release`. But, when compiling with
+# `--source 8`, it is still used for this purpose by
+# `com.sun.tools.javac.file.JRTIndex`.
+#
+# The build generates `jdk.compiler/com/sun/tools/javac/resources/ct.java` from
+# this file, which is then used at runtime.
+
 apple.laf.*: hidden
 apple.security.*: hidden
 com.apple.eawt.*: hidden