To set the selection mask to only select top level nodes (roots)

1. Click the Select by hierarchy and combinations icon in the Status Line (toolbar).
   
2. Set the selection mask to Root.

To set the selection mask to only select only nodes without children (leaves)

1. Click the Select by hierarchy and combinations icon in the Status Line (toolbar).
   
2. Set the selection mask to Leaf.

To select all nodes under the selected node

1. Select Edit > Select Hierarchy.