The standard Redeem Link contains the recipient's email address at the end of the Redeem Link. This is what makes it versatile and usable in many scenarios.
We keep track of those who have received codes using cookies in the web browser to prevent someone from changing the redeem link to include another email address in order to receive another unique code from your Coupon Carrier account. But this isn't entirely secure since the recipient can remove all their cookies in their web browser to prevent us from keeping track of the assigned code.
If your unique codes are sensitive and it's important that someone with technical skills can't manipulate the redeem link in order to receive multiple unique codes you can enable Secure Redeem Links.
When enabled you can't use the standard insecure links for security reasons. Instead you have to pre-generate all the links for each subscriber. This means that you can't use the secure links in all scenarios. But if you have the ability to generate the links in your code base or via another integration then you should use them.
A secure redeem link is encrypted and unique to the requested email address and can't be modified by the recipient.