Add is ad component field to fenced frame config.

spec.bs

@@ -983,7 +983,17 @@ A <dfn export>fenced frame config</dfn> is a [=struct=] with the following [=str
 
   : <dfn>embedder shared storage context</dfn>
   :: null, or a [=string=]
+
+  : <dfn>is ad component</dfn>
+  :: A [=boolean=]. Defaulting to false.
 </dl>
+
+  Note: When true, this [=fenced frame config=] reprsents an ad component. An ad component can be
+  used to construct ads composed of multiple pieces. See the <a
+  href=https://github.com/WICG/turtledove/blob/main/FLEDGE.md#34-ads-composed-of-multiple-pieces>Protected
+  Audience explainer</a>. For an ad component, event reporting is handled differently. See the <a
+  href=https://github.com/WICG/turtledove/blob/main/Fenced_Frames_Ads_Reporting.md#support-for-ad-components>Fenced
+  Frame Ads Reporting explainer</a> that describes this.
 
 <h4 id=fenced-frame-config-instance-struct>The [=fenced frame config instance=] [=struct=]</h4>
 
@@ -1026,6 +1036,9 @@ A <dfn export>fenced frame config instance</dfn> is a [=struct=] with the follow
 
   : <dfn>embedder shared storage context</dfn>
   :: null, or a [=string=]
+
+  : <dfn>is ad component</dfn>
+  :: A [=boolean=]. Defaulting to false.
 </dl>
 
 <div algorithm>
@@ -1110,6 +1123,9 @@ A <dfn export>fenced frame config instance</dfn> is a [=struct=] with the follow
 
     : [=fenced frame config instance/embedder shared storage context=]
     :: |config|'s [=fenced frame config/embedder shared storage context=]
+
+    : [=fenced frame config instance/is ad component=]
+    :: |config|'s [=fenced frame config/is ad component=]
 </div>
 
 Each [=browsing context=] has a <dfn for="browsing context">fenced frame config instance</dfn>,