CorrelationSets

Are used to correlate invoce, receive, and reply activities.

The correlation sets must first be declared:

<correlationSets 
       xmlns:cor="http://example.com/supplyCorrelation.wsdl">
    <!-- Order numbers are particular to a customer, 
         this set is carried in application data -->
  <correlationSet name="PurchaseOrder"
            properties="cor:customerID cor:orderNumber"/>
    <!-- Invoice numbers are particular to a vendor, 
         this set is carried in application data -->
  <correlationSet name="Invoice"
            properties="cor:vendorID cor:invoiceNumber"/>
</correlationSets>

so they can then be used

<receive partnerLink="Buyer" portType="SP:PurchasingPT" 
         operation="AsyncPurchase" 
         variable="PO">
   <correlations>
     <correlation set="PurchaseOrder" initiate="yes">
   </correlations>
</receive>

<invoke partnerLink="Buyer" portType="SP:BuyerPT" 
       operation="AsyncPurchaseResponse" inputVariable="POResponse">
   <correlations>
     <correlation set="PurchaseOrder" initiate="no" pattern="out">
     <correlation set="Invoice" initiate="yes" pattern="out">
   </correlations>
</invoke>

The initiate="yes" means that the correlation is being started (new instance).
The pattern tells us if the correlation applies to the outbound (request) message, the inbound (response) message, or both.