When you are creating a new question, you select the field and then you go to select the model, you will see a check box that says do not use model.
The do not use model option is if you run out of models when you add a new field, you can use this instead. It is a way to add more questions when you have no models left.
If a question already has a model on it and you select do not use model, the information will not be lost.
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