Layerstag in either the @DISPLAY or CreateDisplay() commands.
Please note also that it is generally not advised to switch between layered and non-layered modes because both modes are not really compatible with each other. Thus, the best idea is certainly to define whether or not a display should use layers at display creation time (e.g. when calling @DISPLAY or CreateDisplay()) and then stick to this decision. Mixing layered and non-layered sections in the same display is really only recommended if you know exactly what you are doing.
See Layers introduction for details.