At any time there is always a selected control, the one you are working with. It is distinguished by eight red rectangles that allow you to resize it in all directions.
The selection can be more than just one control. This is useful for aligning a set of controls. Suppose this situation:
Here, we have one control selected. To align the three text fields, you click in a second control (in this case "Text2") holding pressed the SHIFT key. This indicates the editor that you do not want to select a new object but to extend the selection to include both objects. You will see a display similar to this:
The second (most recently selected control) has 8 full red handles, and the other has 8 empty handles, meaning it is seleted, but no longuer the "dominant" control.
To align the two controls you just press the alignment buttons
at the top of the editor. For instance if you want the editor to align the
controls using the left margin you press the button 
that as avertical bar at the left side. This will produce a display like this:
Note that the editor moved the "Text3" control to the position of the dominant control "Text2", that has the full red handles. This is a general principle that applies to selections of several controls, not only two.
When you have several controls selected, clicking in a selected control and holding the SHIFT key will deselect the control. You can change the dominant control by clicking on any of the selected controls (without any SHIFT key).
Note that the coordinates shown in the status bar are the coordinates of the whole selection.