Categories Support in OEM Version

Hi i have seen in the new OEM Version from NACAMAR that there ist a new Field Categories in the KMC.

But how can i set the Categories when i create a new mix?
In the api there is no field named Categories, is this only visible in the KMC?

My example works but no Categories ar added.

$mix = new KalturaMixEntry();
$mix -> name = $title;
$mix -> userId = $user;
$mix -> description = $description;
$mix -> categories = $cat;
$mix -> tags = "videomix";

$mix = $client -> mixing -> add($mix);
$entryId = $mix->id;

Thanks for any ideas how to add categories in a new mix :-)