USE:
XPath expressions are used to operate on XML messages.
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//E1EDK03[IDDAT='001']/UZEIT/text() to find a field UZEIT from node E1EDK03 where other field in this node, IDDAT will contain value '001'
//*:/E1EDK03/UZEIT/text() - to find a field UZEIT from node E1EDK03 - if it would have an unknown prefix
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//*:PostInvoice/transferId/text() or
/*[local-name()="PostInvoice"]/transferId/text()
Examples for non-XML payloads (like EDIFACT) are described on this page.
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