Those are probably check verify transactions. They use the same network that debit cards use. I’m sure Walmart pays for that. I’d still rather type in a PIN than carry a check book and write a novel at the checkout. Or maybe you don’t have to make out the check since it’s superfluous at that point. I assume you have to show ID too.
You don’t have to fill out the check or sign it. The machine fills it out for you, and you get the check back along with your receipt. You don’t have to show ID if you use the system often enough. I don’t know what that limit is. The system is quick and doesn’t require a PIN. I have no idea what the whole thing costs Walmart.