The default ways ( toolbar button or '-' key) select all the (non locked) graphic objectstouched by the rectangle defined by a click+drag+release of the mouse.
All the graphic objects that are (even partially) touched by the rectangle will be selected.
The alternative way ( toolbar button or Ctrl+'-' keys) select all the (non locked) graphic objectscontained in the rectangle defined by a click+drag+release of the mouse.
All the graphic objects that are fully contained in the rectangle will be selected.
By keeping pressed the Ctrl key during an area selection you will toggle the selection status of the touched/contained objects.
Navigation window
Using the navigation dialog ( toolbar button or Ctrl+N keys) you can navigate in the current window graphic object tree and select the desired one.
Extended selection dialog
Using the Extended selection dialog ( toolbar button or S key) you can select the object you want and place them in the right Z-order.