z-index property specifies the stack order of an element. An element with greater stack order is always in front of an element with a lower stack order.
You can change
z-index and order of elements by dragging them by the handle (see picture below). In this case you are changing
z-index and also the order of option sets.
In this example, Frame will be the first option set and also the lowest element of the stack. As you see in the image below, wheels and tires are above the frame and this is incorrect.
What if you want Frames as your first option set but still set
z-index correctly? You can set
z-index manually (see image below). In this case you need to set frames
z-index to 3 and wheels and tires
z-index to 2.