Manfred,
Download and extract the XML schema used in the Form Template.
To retrieve data from the XML, use the following structure:
Enclosed in square brackets ("[ ]"), with a dash ("-") to separate the XML hierarchy elements.
For example, the below would be used in the e-mail which contains a Customer Invoice, within a span tag.
<span>[CustomerInvoice-BillFromParty-FormattedAddressDescription]</span>
Note that it's not always self-explanatory. For example, this is an address line for a Sales Order e-mail. Note that it starts with "FormPurchaseOrder...", which is why you do need to look at the XML schema.
[FormPurchaseOrder-ProductRecipientParty-FormAddress-FirstPostalRegulationsCompliantAddressLine-Description]