What is suitable for testing is a separate feature, that:
- Is big enough to cause a decent shift in an important metric;
- Is small enough so you can easily locate it;
- Is close to an important conversion point, so you can be sure it affects it;
- Is on a critical user path so it is worth wasting your team's time on it;
- Doesn't require a huge additional development for an A/B test itself.
Payment or signup forms are perfect candidates for A/B testing. Improving all the communications across users' journey - not that much.