Objects are painted on the page according to the stacking order. This means that objects can partially or wholly obscure other objects in the same location. For example, in Figure 8.16(a) the green rectangle is at the bottom (back) of the stack (z=0), the blue circle is in the middle of the stack (z=1) and the yellow triangle is at the top (front) of the stack (z=2). The stacking order has been reversed in Figure 8.16(b), with the green rectangle now at the top (front) of the stack (z=2) and the yellow triangle now at the bottom (back) of the stack (z=0).
Each new object is automatically added to the front when it is created, but objects can also be moved to the front either using the move to front button or by selecting Edit->Move to Front (Ctrl-F). Alternatively you can move an object up the stacking order using Edit->Move Up.
A selected object can be moved to the back of the stack (so that other objects in the same location obscure it) either by clicking the move to back button or by selecting Edit->Move to Back (Ctrl-B). Alternatively you can move an object down the stacking order using Edit->Move Down.
If objects don't overlap or if they don't have a fill colour, the stacking order may not be immediately apparent, but the order is important when you're defining frames for the flowfram package. You can cycle the selection in reverse order from front to back of the stack using the Navigate->Select menu item.