It's not so bad, really. If it's documented in your medical records while you're still on active duty there's a presumption that it's service connected. It's when you go and try to get disability for something that begins after separation or retirement that you have a bit of a hurdle to prove it was your military time that caused the ailment. If you don't have any documented tinnitus, retire, and then go in ten years later to try to get disability for the ringing you have in your ears now (but have been exposed to a lot of loud noises since you've retired), they're not going to be inclined to pay you disability for it. It's very important to get your medical issues documented and addressed BEFORE you separate or retire.