diff --git a/doc/03 - Configuration.md b/doc/03 - Configuration.md
index 73a906d..e6fbf70 100644
--- a/doc/03 - Configuration.md	
+++ b/doc/03 - Configuration.md	
@@ -7,10 +7,16 @@
 # config/packages/bentools_webpush.yaml (SF4) 
 bentools_webpush:
     settings:
+        # subject: <URI>
         public_key: 'your_public_key'
         private_key: 'your_private_key'
 ```
 
+The subject is optional, with a fallback being `router.request_context.host`. 
+You may need to define it explicitly if you are working in a CLI context.
+
+Note that Apple requires the subject to be an URL or a mailto URL.
+
 #### Update your router:
 ```yaml
 # app/config/routing.yml (SF3)