The default rule is a final rollout rule for any users that are not individually targeted and do not match any of your configured targeting rules.
As with other rules, you can choose to serve a specific variation, or apply a percentage rollout to any remaining users.
For a straightforward percentage rollouts, you can simply use the default rule for a feature flag. For example, to roll out the
true variation to 50% of your users, set the default rollout rule to
true 50% and