Clarify what compliant implementations are

specification/modules.rst

@@ -18,7 +18,10 @@ Modules
 
 Modules are parts of the Client-Server API which are not universal to all
 endpoints. Modules are strictly defined within this specification and
-should not be mistaken for XEP or equivalent extensions from other protocols
+should not be mistaken for experimental extensions or optional features.
-- in order for an implementation to be compliant with the Client-Server
+A compliant server implementation MUST support all modules and supporting
-specification it MUST support all modules and supporting specification.
+specification (unless the implementation only targets clients of certain
-The exception being clients, which are governed by `Feature Profiles <#feature-profiles>`_.
+profiles, in which case only the required modules for those feature profiles
+MUST be implemented). A compliant client implementation MUST support all
+the required modules and supporting specification for the `Feature Profile <#feature-profiles>`_
+it targets.