diff --git a/index.bs b/index.bs
index 89ae6e21..9cef4301 100644
--- a/index.bs
+++ b/index.bs
@@ -474,8 +474,15 @@ An implementation may define extension modules. These must hav
[=module name=] that contains a single colon ":
" character. The
part before the colon is the prefix; this is typically the same for all
[=extension modules=] specific to a given implementation and should be unique for a
-given implementation. Such modules extend the [=local end definition=] and [=remote end definition=]
-providing additional groups as choices for the defined [=commands=] and [=events=].
+given implementation.
+
+Other specifications may define their own WebDriver-BiDi modules that extend the protocol.
+Such modules must not have a name which contains a colon (:
) character,
+nor must they define [=command names=], [=event names=], or property names that contain that character.
+
+Authors of external specifications are encouraged to to add new modules rather than extending
+existing ones. Where it is desired to extend an existing module, it is preferred to integrate the
+extension directly into the specification containing the original module definition.
## Commands ## {#commands}