Everyone has their own preferred brand of soda. For the majority of people, it comes down to Coke or Pepsi, although other options are, of course, available. All soda companies are at war with each other, although we've never seen that war depicted like this.
We may never know what drove this customer to create a cartoon version of the soda wars on the back of their receipt, but it at least provided some entertainment to the waiter or waitress who found it.