Pay As You Go: usage cap
Usage cap is a feature of Project X that allows you to protect your customers from huge bills when using Pay As You Go pricing.
Imagine: one mistake in the client code and your customer could make a ton of unexpected API calls to your API, resulting in a huge invoice at the end of the month.
To avoid that, you can impose a maximum usage limit on the number of API calls, so that your customers can be sure that, even in a case of a malicious user, code mistake in their application, or alike, they never exceed the usage limit and never get a ridiculously huge bill.
Project X will apply Pay As You Go model, track the actual usage, and cut the access before the usage gets too high.