What is the Gdufa fee?

Can you elaborate on the Gdufa fee? I'm curious to understand its purpose and how it applies to the pharmaceutical industry. Is it a mandatory fee that drug manufacturers need to pay, and if so, what are the specific circumstances that necessitate its payment? Additionally, how does the revenue generated from this fee contribute to the FDA's operations, and are there any exemptions or discounts available for small or low-income drug manufacturers?