As you said, what cle described is a gradual rollout. You can do both at the same time though -- simply start your test on X% of the population and then roll it

As you said, what cle described is a gradual rollout. You can do both at the same time though -- simply start your test on X% of the population and then roll it out to more people as you run your test. This is also a great way to run long-term holdouts.