A Z-Index is an internal property of a Graphic in a GraphicCollection that defines the stack order of the Graphic. A Graphic with a higher Z-Index value always draws on top of another Graphic with a lower Z-Index value.

By default all Graphic objects in a GraphicCollecton have a Z-Index value of zero (0) unless specifically set in either code-behind or XAML.

If no Z-Index value has ever been set, Graphics are drawn on the Map in the order that they are created in code-behind or XAML. This means the first Graphic in the GraphicCollection is drawn first and the process repeats itself until the last Graphic in the GraphicCollection is draw.

Do not confuse the Z-Index with another common GIS term: Z-Value. A Z-Value in an elevation or terrain model means elevation. In other kinds of surface models a Z-Value could represent density or quantify of a particular attribute.