Any entry can be the heading of a lower level list of entries - called 'descendants' in BrainStorm parlance. Use Promote to move the entry to the heading position on the display.
To move entries to a higher level, shift drag them to the heading. To move them to a lower level, shift drag them to another entry. If the destination is not in view, use Split window to view two parts of the model at the same time then drag and drop entries between them. For simple restructuring, use cut and paste. Or use Mark with Throw or Retrieve for more bulky jobs.