[3dtrialpractice]

I read this in the manual:
Use for Disabled: This option adds an extra control over the environment. If any
of the MXI/ HDR channels are disabled, you have the option to use the Sky Dome,
Physical Sky or None to replace the disabled channels. For example, if you disable the
IBL Illumination channel and set “Use for Disabled” to Physical Sky, it will provide the
illumination for the scene.

But in maya we dont need this option as we can Enable BOTH Sky/Dome and IBL at the same time and what ever channel in IBL is unchecked then the SKY/DOME will be rendered instead!

-edit: at first i said this option was missing.. now I understand how it works..