Enabling %1$scross-domain tracking (additional setup required)%2$s allows you to track users across multiple properties you own (such as example-1.com and example-2.com as a single session. It also allows you fix an issue so that when a user has to go to an off-site hosted payment gateway to finish a purchase it doesn't count it as referral traffic from that gateway but maintains the visit as part of the same session.) It is required that the other site includes a Google Analytics tracker with the same UA Code.