ConstraintLayout: What does `layout_constraintLeft_creator` do in xml? : <p>Example code:</p>

<pre><code> &lt;EditText
        android:id="@+id/msg_type"
        android:layout_width="0dp"
        android:layout_height="40dp"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginStart="8dp"
        android:hint="Input message"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintHorizontal_bias="0.75"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toLeftOf="@+id/btn_chat_send"
        tools:layout_constraintBottom_creator="1"
        tools:layout_constraintLeft_creator="1"
        tools:layout_constraintRight_creator="1"/&gt;
</code></pre>

<p>What does <code>tools:layout_constraintRight_creator="1"</code> do here? There aren't any document explaining these things.</p>
