What is the AssignMessage policy and what can it do?

<!-- Set headers on the outgoing request to backend -->
<AssignMessage name="AM.SetBackendHeaders">
  <AssignTo createNew="false" type="request"/>
  <Set>
    <Headers>
      <Header name="x-internal-caller">apigee-proxy</Header>
      <Header name="x-correlation-id">{request.header.x-correlation-id}</Header>
    </Headers>
  </Set>
  <Remove>
    <Headers>
      <Header name="x-api-key"/>  <!-- strip the key before forwarding -->
    </Headers>
  </Remove>
</AssignMessage>

<!-- Create a custom error response -->
<AssignMessage name="AM.CustomError">
  <AssignTo createNew="true" type="response"/>
  <Set>
    <StatusCode>403</StatusCode>
    <ReasonPhrase>Forbidden</ReasonPhrase>
    <Headers>
      <Header name="Content-Type">application/json</Header>
    </Headers>
    <Payload contentType="application/json">{
      "error": "access_denied",
      "message": "You do not have permission to call this API."
    }</Payload>
  </Set>
</AssignMessage>

<!-- Change the request verb and path -->
<AssignMessage name="AM.RewritePath">
  <AssignTo createNew="false" type="request"/>
  <Set>
    <Verb>POST</Verb>
    <Path>/v2/orders</Path>
  </Set>
</AssignMessage>