This will allow PDFShift to go beyond your plan's limit and you will be billed for the extra credits on a per-credis based.
For instance, if you are on the Business plan at 25.000 credits per month, you can set the overage at 27.500 credits.
During the month, PDFShift will send you notifications of your credit usage at 50%, 80%, 90% and 100% of your plan (25.000 credits), but once it reaches 100%, it won't block the requests, and will continue to generate documents until the credits usage reaches 27.500.
In that case, at your plan renewal, you'll receive an invoice of 99$ for your current plan, and another invoice of 50$, related to 2500 extra credits x 0,02$. (Each plan has a different extra credit cost).
We recommend you to enable this when you know you'll exceptionally go beyond your plan's limit. You are free to change this value anytime you want too.