This is a letter to be used when you need to escalate things a bit. It's not a final letter, as it leaves open several further, stronger options, but it is definitely a step up from the first level.
This is another letter you can use for a "3rd request" for payment letter. The wording on this is stepped up significantly from the 2nd request letter. In this case, the debtor is advised that you have no alternative but to begin holding orders.