Reset object_type in tracker.slot

How to reset object_type in tracker.slot? Or When to reset object_type in tracker.slot?

Please answer about my question. Thanks.

How can I extract entity role, entity group from slot

As learning data of nlu.md file:

Give me a [small]{"entity": "size", "group": "1"} pizza with [mushrooms]{"entity": "topping", "group": "1"} and a [large]{"entity": "size", "group": "2"} [pepperoni]{"entity": "topping", "group": "2"}.

How can I extract entity role, entity group from tracker slot in...