Currently the selection node (vtkMRMLSelectionNode) only changes state on data load (at least this is the case for volumes). This is confusing, partly because of the naming, partly because of the PropagateVolumeSelection method. When we use the method, we would expect that the 'selection' is propagated, not the lastly loaded nodes shown.
An example:

• Load volumes, and a labelmap (an atlas)
• View all the loaded data in different ways
• Hide labelmap
• Programmatically showing a volume like this: set active selection in the selection node, then propagate volume selection
• What happens is that the atlas gets shown on top of the intended volume, so the opposite happens to what I would expect (i.e. the atlas can only be seen, and the volume is not)

It would be great to think about this.
One possibility (that I talked about with Jc), is to actually set the selection of the volumes when the user changes slice view layers.