The sample data is a postilion structured data field which uses kind of a TLV (tag length value format).
221ThirdPartyBillPayment3125
<ThirdPartyBillPayment>
<BillPaymentRequest>
<ReferenceId>1111111111</ReferenceId>
</BillPaymentRequest>
221ThirdPartyBillPayment
Here 2 is the length of length (21), 21 is the length of the tag ThirdPartyBillPayment
3125
<ThirdPartyBillPayment>
<BillPaymentRequest>
<ReferenceId>1111111111</ReferenceId>
</BillPaymentRequest>
</ThirdPartyBillPayment>
Here 3 is the length of length (125), 125, is the length of data to follow.
You could write code to get access to xml iteratively for all thats available in structured data and then parse out the xml data within.
Or
You could ask Postilion for the dtd/schema for the xml used in their structured data iso field and use jaxb to access the data.
It will boil down to a name value pair
ThirdPartyBillPayment= <ThirdPartyBillPayment><BillPaymentRequest<ReferenceId>1111111111</ReferenceId></BillPaymentRequest>
</ThirdPartyBillPayment>