How does BotML/Dialog Flow know to ask for missing Entities values?

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In the delivered skill example of (PizzaKing) how does the system know to ask for missing entity value?

(https://docs.oracle.com/en/cloud/paas/digital-assistant/tutorial-skill/files/complete-dialog-flow.txt)

So if I enter "I want a veggie pizza at 8:00pm", how does the system know that the pizza size is missing and prompt for it?

Also how can I set the flow so that if a Cheese pizza is being ordered then don't ask for size as Cheese pizza is only available in large size?

This is a bit different then conventional programming languages as everything there has to be programmed to be evaluated with if (variable not set)-then(prompt for value).