There's a number of menu pictures available on these boards (found near the end of almost all the restaurant review threads) - it's the set menus which are included and they usually have a little half board logo next to them.
However - you are technically able to choose from anything on any menu - it's just that if you choose something that's not a set menu then you have to pay any difference in cost between the value of your voucher and the cost of your meal (set menus are always free regardless of their price on the menu).