Looking at the data manager we find
node->SetSelected(..) in void QmitkDataManagerView::NodeSelectionChanged(const QItemSelection& /*selected*/, const QItemSelection& /*deselected*/).
This sets the selected property of the data node so that other plugins can easily find out if a node has been selected. This is currently used in T25483.
Since we use the new selection concept - also with a focus on a more local selection - we do not maintain the selected property anymore (apart from the data manager).
We should investigate if this property is still needed / used by other plugins and if so how we want to maintain the selected property using the new selection concept.