A Core deposit is a form of deposit you pay until returning your old part. If you don't have the core at the time of purchase, you must pay the core charge. That charge is refunded to you when you return the core.
The ring gear has teeth which engage with the starter motor allowing the engine to crank over and start. Repeated engagement with the starter results in worn or broken ring gear teeth. Once this is diagnosed, the ring gear must be replaced. ATP ring gears are designed to fit and function just like the original part to ensure durability and long life.