Z-order refers to the order of multiple overlapping objects in a diagram.
The actual number assigned to a particular place in the Z-order is sometimes known as the z-index.
An object with a Z-index of 1 will appear behind an object with a Z-index of 2 or greater. In other words, 1 is at the bottom of the stack or the furthest back.
In SmartDraw, there are two kinds of Z-orders to consider: shapes and layers.
Changing the Z-Index of Shapes
Any group of symbols can arranged using the commands "Send to Back" and "Bring to Front". This is outlined in this article about changing the order of shapes.
Clicking on Shape B and choosing Bring to Front in the Home tab will change its Z-index of 2 to a Z-index of 3. It will also move shift Shape A down into Z-index 2.
Changing the Z-Index of Layers
When you're dealing with layers, you can modify their Z-order using the Manage Layers option under Page->Layers->Manage Layers or you can click the layers at the bottom left of your workspace and bring up the Manage Layers dialog.
Here, you can move a layer up or down and change its Z-index.