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Different interpretations of a tab/column or tree/column, as well as some other, completely unrelated structures are all possible results of applying the layout XML to an XSL stylesheet. Below is an excerpt of a "tab/column" structure XML.
Tab Column
The default (and most common) structure utilized is tab-column.
Tree
Some institutions prefer to expose a user's layout in the form of a file-browser like navigable tree.
Deck of Cards
Some mobile devices have limited screen real estate and benefit from a collapsed navigation display.