i'm trying to export a large cluster to an XML string which is going to be saved afterwards. Another VI should be able to import the XML - but where to get the data type? The "unflatten from XML" function needs this information.
Simply use the same data type that you wired into the Flatten To XML node. For example, you could use a constant created from the flattened data type, or wire it directly from the control that you used for flattening the data. There is no single "right" answer.