Hi bobt,
it is a subscription. You subscribe and then you pay every month, until you cancel your subscription. You may choose the auto-refill option on signup. That means that you pay $40 every month, or the equivalent in your country's currency, and if you surpass
4 million characters, you automatically start a new monthly subscription at that time, which replaces your old one. This way you can be subscribed at 4 million characters and won't be cut off if you surpass the volume occasionally. If you surpass the volume
consistently, you will want to consider subscribing at the higher volume, for a lower per-character price.
Let us know if this makes sense,
Chris Wendt
Microsoft Translator