declare variable $input1 as document-node(element(*, xs:untyped)) external;
declare variable $input2 as document-node(element(*, xs:untyped)) external;

<PurchaseOrderLine>
  <LineNumber>
    { $input1/TradePowerRequest/RequestDetail/ItemLineNumber/text() }
  </LineNumber>
  {
    for $RequestDetail in $input1/TradePowerRequest/RequestDetail
    return
      <items>
        {
          if($RequestDetail/ItemNumber/@source = "UI") then
            <item1>
              { $input2/DetailAvailability/DisplaySequence/text() }
            </item1>
          else ()
        }
      </items>
  }
</PurchaseOrderLine>