Selecting Feed Rectangles
There are three techniques for selecting rectangles:
Uncovering Rectangles
Left-click a Rectangle to selects one rectangle. When you continue clicking, you uncover more rectangles that share the same position but are hidden behind another rectangle.
Selecting multiple Rectangles
Working with more than one Feed Rectangle at a time is very common. Left-clicking and dragging the mouse will selects all rectangles which touch the area dragged over by the mouse.
Toggling through Rectangles
Ctrl + clicking Feed Rectangles toggles their selection, allowing you to extend the selection across areas which are not next to one another.