The guild ranks up to HR8 let you take on a different tier of quests. Once you get into G Rank, the ranks themselves don't matter as much, but the little symbols by your name do. They tell what rank of G you are. After you beat G3, it will move onto little crowns and to advance crowns, you have to keep doin missions to decrypt scrolls, which unlock missions. After you hit HR 100, all the HR restricted missions will be unlocked, but there should still probably be some other quests that unlock in special ways. Once you get gold crown, you've unlocked all the scrolls and the only quests that might be left are the ones that have to be unlocked by hunting 50 of a certain monster classification (Fanged Beast, Snake Wyvern, etc.) and other requirements like that.