The Workplace Forms Designer allows the form author to use an XML schema to automatically generate the instance data for an XForms model. The data nodes are needed to allow drag-and-drop associations to be created between the data and user interface components. You can drag from an existing UI control onto the data to make the association, or you can drag the data onto the form design canvas to cause the creation of a UI control that will then be bound to the data node. Either way, in XForms the association is made between instance data nodes and UI controls.
This is true even when the XML schema contains an element whose content is a choice. When this happens, the Workplace Forms Designer generates data according to the content of all possible choices. This allows form design to proceed for each of the possible choices. The expectation from XForms is that the form author will make all of the possible choices be non-relevant except the one actually chosen by the end-user. For example, if you have a choice of address block based on country selection, once you select "UK" then you get the UK address block, and once you select "US" then you get the US address block, and so forth. All address blocks other than the one for the selected country become non-relevant.
The use of non-relevance in XForms is significant. It affects two things, but the most important is that all non-relevant nodes are removed from XML data at the start of submission processing. This is important because an XML element with a schema choice content model is not valid until all but one of the choices is removed. This is called "pruning non-relevant nodes" in XForms, and it means that XForms does not expect instance data to be schema valid