Does anyone know how the onboard cafe voucher works? I've never bought one before but I have an upcoming domestic segment where it might be easier given my short connection time

It says up to a $9 value for a snack and a meal. But looking at the menu, snacks are 3-5$ and meals are $7-8. Can you still select any snack plus any meal?

Also does anyone know if you can use the voucher towards upgrade to one of the hot gourmet meals?