diff --git a/google/maps/fleetengine/v1/vehicle_api.proto b/google/maps/fleetengine/v1/vehicle_api.proto
index ab3206ac49..37838c3d5e 100644
--- a/google/maps/fleetengine/v1/vehicle_api.proto
+++ b/google/maps/fleetengine/v1/vehicle_api.proto
@@ -457,7 +457,7 @@ message SearchVehiclesRequest {
   //
   // `EXCLUSIVE` and `SHARED` may not be included together.
   // `SHARED` is not supported when `current_trips_present` is
-  // `CURRENT_TRIPS_PRESENT_UNSPECIFIED`.
+  // `CURRENT_TRIPS_PRESENT_UNSPECIFIED`. `UNKNOWN_TRIP_TYPE` is not allowed.
   repeated TripType trip_types = 9 [(google.api.field_behavior) = REQUIRED];
 
   // Restricts the search to only those vehicles that have updated their
@@ -466,7 +466,8 @@ message SearchVehiclesRequest {
   google.protobuf.Duration maximum_staleness = 10;
 
   // Required. Restricts the search to vehicles with one of the specified types.
-  // At least one vehicle type must be specified.
+  // At least one vehicle type must be specified. VehicleTypes with a category
+  // of `UNKNOWN` are not allowed.
   repeated Vehicle.VehicleType vehicle_types = 14
       [(google.api.field_behavior) = REQUIRED];
 
@@ -620,7 +621,7 @@ message ListVehiclesRequest {
   google.protobuf.Duration maximum_staleness = 8;
 
   // Required. Restricts the response to vehicles with one of the specified type
-  // categories.
+  // categories. `UNKNOWN` is not allowed.
   repeated Vehicle.VehicleType.Category vehicle_type_categories = 9
       [(google.api.field_behavior) = REQUIRED];