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One place this is very useful is in business rules. See the section on templatized URls in business rules for more details. However anywhere that you can use templates in your form: labels, help, hint; display message, etc... the value can either come from the value in an actual form field or from _data values that are not in any way bound to the name of a control.
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See default field values to understand the precedence that XML documents sent in the Post request to form B take over the other possible methods of defaulting form field values.
See Initializing Forms with XML Documents for more information.