Layouts can be used with other kind of objects than documents.
The field definition has to match a document property for which setters and getters will be available, or the value property must be passed explicitly for the binding to happen. Depending on the widget, other kinds of bindings can be done.
The field definition can contain the complete binding itself and be independent from the value passed to the tag. It just needs to be formatted like an EL expression, for instance: #{myBinding}.