yo, I have a categorical slot in my bot:
but then I dont understand why DIET is filling my slots from nlu fallback intent 
Hello @arkaprabha-majumdar
You might either not have enough training data or a too-low number of training epochs for DIET. DIET doesn’t know what intents or entities mean and it has to learn what belongs to what from the training data. The less data you have, the more epochs you need to train.