diff --git a/automotive/vehicle/aidl/android/hardware/automotive/vehicle/IVehicle.aidl b/automotive/vehicle/aidl/android/hardware/automotive/vehicle/IVehicle.aidl
index 47fc54b..c896d14 100644
--- a/automotive/vehicle/aidl/android/hardware/automotive/vehicle/IVehicle.aidl
+++ b/automotive/vehicle/aidl/android/hardware/automotive/vehicle/IVehicle.aidl
@@ -190,6 +190,14 @@
      * what the sampleRate specified in {@code options}, the timestamp for
      * the timestamp is updated 10 times/s.
      *
+     * If a property is unavailable for reading because it depends on some power
+     * state which is off, property change event may not be generated until the
+     * property becomes available. For ON_CHANGE property, if the property
+     * changes from NOT_AVAILABLE to OKAY for reading some or all area(s), for
+     * each area that becomes available for reading, one property change event
+     * must be generated. The event must contain the current value for the area
+     * and must have {@code AVAILABLE} status.
+     *
      * @param callback The subscription callbacks.
      *    {@link IVehicleCallback#onPropertyEvent} would be called when a new
      *    property event arrives.
diff --git a/automotive/vehicle/aidl/android/hardware/automotive/vehicle/StatusCode.aidl b/automotive/vehicle/aidl/android/hardware/automotive/vehicle/StatusCode.aidl
index 23019ca..35080db 100644
--- a/automotive/vehicle/aidl/android/hardware/automotive/vehicle/StatusCode.aidl
+++ b/automotive/vehicle/aidl/android/hardware/automotive/vehicle/StatusCode.aidl
@@ -24,7 +24,11 @@
 enum StatusCode {
     OK = 0,
     /**
-     * Try again.
+     * Caller should try again.
+     *
+     * This code must be returned when an ephemeral error happens and a retry
+     * will likely succeed. E.g., when the device is currently booting up
+     * and the property is not ready yet.
      */
     TRY_AGAIN = 1,
     /**
@@ -32,9 +36,22 @@
      */
     INVALID_ARG = 2,
     /**
+     * The property is currently unavailable and will be unavailable unless
+     * some other state changes.
+     *
      * This code must be returned when device that associated with the vehicle
      * property is not available. For example, when client tries to set HVAC
      * temperature when the whole HVAC unit is turned OFF.
+     *
+     * The difference between this and TRY_AGAIN is that if NOT_AVAILABLE is
+     * returned for a property, it will remain NOT_AVAILABLE unless some other
+     * state changes. This means a retry will likely still return NOT_AVAILABLE.
+     * However, for TRY_AGAIN error, a retry will likely return OK.
+     *
+     * When subscribing to a property that is currently unavailable for getting.
+     * VHAL must return OK even if getting/setting must return NOT_AVAILABLE.
+     * VHAL must not generate property change event when the property is not
+     * available for getting.
      */
     NOT_AVAILABLE = 3,
     /**