The Z Order bar indeed does display accordingly to the selected object.
When you click in the layout view where there is not an instance of an object, the Z order bar's title will indicate "Showing all".
And when you select an instance of an object "covering"/in collision with other instances/object types, all those will appear in the bar, and the title will indicate "Showing subset".
At last, holding the tab key pressed and clicking on instances that are covered, allow you to select instances at a different Z order that a single regular selection would not select as being underneath a covering instance.