Optional to use filter in subscribe request. Explanation added.

spec/VISSv2_Transport.html

@@ -1039,6 +1039,8 @@ <h2>Subscribe</h2>
 	  <li>range: notifications are issued when the value is in the specified range,</li>
 	  <li>curvelog: notifications are issued when the buffer is full, and then processed according to the curve logging algorithm.</li>
 	  </ul>
+	  If none of the above trigger condition variants is specified, 
+	  then a notification will be issued whenever the underlying vehicle system supplies a new data point to the server. 
             </p>
 	<table class="simple">
  	<thead>
@@ -1047,7 +1049,7 @@ <h2>Subscribe</h2>
 	 <tbody>
 	   <tr><th rowspan="5"><dfn data-dfn-type="dfn" id="dfn-subscriberequest">subscribeRequest</dfn></th><td><a href="#dfn-action">action</a></td><td><a href="#action-def">Action</a></td><td>Yes</td></tr>
 	   <tr><td><a href="#dfn-path">path</a></td><td>string</td><td>Yes</td></tr>
-	   <tr><td><a href="#dfn-filter">filter</a></td><td>string</td><td>Yes</td></tr>
+	   <tr><td><a href="#dfn-filter">filter</a></td><td>string</td><td>Optional</td></tr>
 	   <tr><td><a href="#dfn-authorization">authorization</a></td><td>string</td><td>Optional</td></tr>
 	   <tr><td><a href="#dfn-requestid">requestId</a></td><td>string</td><td>Yes</td></tr> 
 	 </tbody>