A/B testing is used when you want to compare multiple versions of an app to see
which is better. You can, for example, give two versions 50% of the traffic. Split by IP address, that way once a user starts getting one version they get that version for every subsequent request. You can then define some objective measure to determine which version is performing better.

You can also do A/B testing and split the traffic with a cookie. The cookie would
determine which version the user gets. You might define a set of beta testers who will get the latest version. When they log in, you assign them the cookie.