diff --git a/docs/protocol-extensions.rst b/docs/protocol-extensions.rst
index 5f41ee3ed..976fec3d7 100644
--- a/docs/protocol-extensions.rst
+++ b/docs/protocol-extensions.rst
@@ -172,7 +172,10 @@ instead of being blank.
 The motivation for this is that many modern shells will show completions in a
 block of lines under the cursor, this causes some of the on-screen text to be
 lost even after the completion is completed, because it has scrolled off
-screen. This escape code allows that text to be restored.
+screen. This escape code allows that text to be restored. If the scrollback
+buffer is empty or there is no scrollback buffer, such as for the alternate
+screen, then the newly inserted lines must be empty, just as with the original
+``SD`` escape code.
 
 The syntax of the escape code is identical to that of ``SD`` except that it has
 a trailing ``+`` modifier. This is legal under the `ECMA 48 standard