diff --git a/envoy/api/v2/core/config_source.proto b/envoy/api/v2/core/config_source.proto
index ea86c2cd..b1e0fa58 100644
--- a/envoy/api/v2/core/config_source.proto
+++ b/envoy/api/v2/core/config_source.proto
@@ -30,12 +30,15 @@ message ApiConfigSource {
     // the v2 protos is used.
     REST = 1;
 
-    // "State of the world" gRPC v2 API, using Discovery{Request,Response} protos.
+    // gRPC v2 API.
     GRPC = 2;
 
-    // "Delta" gRPC v2 API, using DeltaDiscovery{Request,Response} protos.
-    // Rather than sending Envoy the entire state with every update, the xDS server
-    // only sends what has changed since the last update.
+    // Using the delta xDS gRPC service, i.e. DeltaDiscovery{Request,Response}
+    // rather than Discovery{Request,Response}. Rather than sending Envoy the entire state
+    // with every update, the xDS server only sends what has changed since the last update.
+    //
+    // DELTA_GRPC is not yet entirely implemented! Initially, only CDS is available.
+    // Do not use for other xDSes. TODO(fredlas) update/remove this warning when appropriate.
     DELTA_GRPC = 3;
   }
 
diff --git a/envoy/api/v3alpha/core/config_source.proto b/envoy/api/v3alpha/core/config_source.proto
index d470fdef..5da66d99 100644
--- a/envoy/api/v3alpha/core/config_source.proto
+++ b/envoy/api/v3alpha/core/config_source.proto
@@ -30,12 +30,15 @@ message ApiConfigSource {
     // the v2 protos is used.
     REST = 1;
 
-    // "State of the world" gRPC v2 API, using Discovery{Request,Response} protos.
+    // gRPC v2 API.
     GRPC = 2;
 
-    // "Delta" gRPC v2 API, using DeltaDiscovery{Request,Response} protos.
-    // Rather than sending Envoy the entire state with every update, the xDS server
-    // only sends what has changed since the last update.
+    // Using the delta xDS gRPC service, i.e. DeltaDiscovery{Request,Response}
+    // rather than Discovery{Request,Response}. Rather than sending Envoy the entire state
+    // with every update, the xDS server only sends what has changed since the last update.
+    //
+    // DELTA_GRPC is not yet entirely implemented! Initially, only CDS is available.
+    // Do not use for other xDSes. TODO(fredlas) update/remove this warning when appropriate.
     DELTA_GRPC = 3;
   }