What is a referral code, and how does it work in various programs? Can you explain the benefits of using a referral code for both the person sharing it and the one using it? Additionally, how do businesses incorporate referral code into their marketing strategies to grow their customer base? What are some examples of common rewards, such as discounts, cash bonuses, or in-game items, that users can receive through these programs? Lastly, how can individuals maximize their earnings or rewards by effectively sharing referral codes with others?