Question about "additional credits"

Hi everyone, I have a simple question, but I’m not entirely clear on the answer. I was checking the “Usage” section—just to clarify, I’m on an annual plan. I see “monthly credits,” “additional usage,” and “additional credits.”

Could someone please explain what exactly the additional credits are?

If I use up all my monthly credits, what happens next?
Do I automatically get charged extra on my credit card (based on my additional usage), or do I first use up any available additional credits before incurring extra charges?

Thanks in advance!