The z-index property specifies the stack order of a creative element. An element with a higher stack order number always appears in front of an element with a lower one.
The z-index value of a DoubleClick Rich Media ad is set to 1000000 by default. You can change the value in the Preview tab of a creative. The field is called z-index.
See also the z-index section in Set Rich Media creative properties in the DCM Help Center.