What is the ExtractVariables policy and how does it work with flow variables?

<!-- Extract from JSON request body -->
<ExtractVariables name="EV.ExtractBody">
  <Source>request</Source>
  <JSONPayload>
    <Variable name="orderId">
      <JSONPath>$.order.id</JSONPath>
    </Variable>
    <Variable name="customerId">
      <JSONPath>$.order.customer.id</JSONPath>
    </Variable>
  </JSONPayload>
</ExtractVariables>

<!-- Extract from URI path pattern: /orders/{orderId}/items/{itemId} -->
<ExtractVariables name="EV.PathVars">
  <Source>request</Source>
  <URIPath>
    <Pattern ignoreCase="true">/orders/{orderId}/items/{itemId}</Pattern>
  </URIPath>
</ExtractVariables>

<!-- Extract from query parameter -->
<ExtractVariables name="EV.QueryParam">
  <Source>request</Source>
  <QueryParam name="customerId">
    <Pattern>{customerId}</Pattern>
  </QueryParam>
</ExtractVariables>

<!-- Now use the variable in a condition or another policy -->
<!-- Example: add the orderId as a backend header -->
<AssignMessage name="AM.InjectOrderId">
  <Set>
    <Headers>
      <Header name="x-order-id">{orderId}</Header>
    </Headers>
  </Set>
</AssignMessage>