when you challenge someone, there is a option SHOW MORE ADVANCED OPTIONS. click on that & you will get it..
rating
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Click show more options. Then go to where it says rated and you can choose two styles of unrated: normal unrated or take-back(you can take back moves that were a mistake-in which some people will play a whole game and then take back all the moves back to the beginning and then abort the game).
Also in more options, you can choose to play chess 960