diff --git a/guidelines/extensions.rst b/guidelines/extensions.rst
index 4574dc3..fac14ca 100644
--- a/guidelines/extensions.rst
+++ b/guidelines/extensions.rst
@@ -5,15 +5,26 @@ API Extensions
 
 This topic document serves to provide guidance on the topic of API extensions.
 
-See also the topic document on :ref:`discoverability`.
+See also the topic documents on :ref:`discoverability` and
+:ref:`interoperability`.
 
 Guidance
 --------
 
-**TODO** Add patches that give a history of current usage of API extensions in
-OpenStack projects.
+API extensions are sometimes used to add custom functionality to single
+deployments of OpenStack. They are not recommended, because when they are
+used, they break interoperability between that cloud and other OpenStack
+deployments.
 
-**TODO** Add patches that list the issues with API extensions.
+If a deployment requires custom behaviors over an HTTP API it should be
+implemented in a service separate from the existing OpenStack services.
 
-**TODO** Add a patch that makes a proposal about whether or not to recommend
-use of API extensions.
+Those projects that support different functionality based on the presence
+of drivers should strive to contain those differences to the values (not keys)
+in representation objects. Having different URLs in a service, based on
+different drivers, breaks interoperability. Where such functionality is
+absolutely required then it is critical that the functionality be discoverable
+via a capabilities API.
+
+.. note:: At this time the standards and best practices for capabilities
+          discovery are undefined.