Add more description

index.html

@@ -818,9 +818,16 @@ <h3>
         <p>
           The [=manifest's=] <code><dfn data-export="" data-dfn-for=
           "manifest">id</dfn></code> member is a <a>string</a> that
-          represents the <dfn>id</dfn> for the application. It is used by user
-          agents and third party entities to uniquely identify the application
-          globally.
+          represents the <dfn>id</dfn> for the application.
+        </p>
+        <p>The |id| is used  by user agents uniquely identify the application
+          globally. When |id| is changed, it's recognized as a new app and
+          can be installed separately. When id is not changed, it can be used
+          to find installed application to perform manifest update.
+        </p>
+        <p>
+          The |id| can also be used by third party entities such as PWA stores
+          to uniquely identify and reference web apps
         </p>
         <p>
           To <dfn>process the `id` member</dfn>, given [=object=]