I noticed this instore the other day. Spoke to an assistant about it. She was saying that it covers virtually everything in the store, and you can mix and match with anything else in the store.
I thought the 3 for £10 deal was good, but 5 for £15 is even better :)
£2.50 off £25 spend of £5 off £50 spend vouchers for farmfoods: https://www.farmfoods.co.uk/Vouchers.php