It's been removed. You don't need to specify it anymore.
Starting with 2013.1, the Music Switch Container support multiple switch or state groups in the same container.
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Go to the Interactive Music Layout (press F10)
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Drag and drop Switch or State Groups in the top section of the Music Switch Container Association Editor
or
Use the >> button to select which state or switch group to add
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Associate the content to switches or states using the bottom pane
For more information, refer to this topic in the Wwise Help:
Wwise Help > Creating Interactive Music > Defining Music Object Playback Behaviors > Defining the Contents and Behaviors of Music Switch Containers > Associating Music Objects with States and/or Switches