Here is Googles writeup on staged rollouts with bolding and italics done by me
Release app updates with staged rollouts
You can release an app update to production using a staged rollout. With a staged rollout, your update reaches only a percentage of your users, which you can increase over time.
Staged rollouts can only be used for app updates to production, not when publishing an app for the first time or creating an alpha or beta release.
User eligibility & targeting
New and existing users are eligible to receive updates from staged rollouts and are chosen at random for each new release rollout.
When setting up a staged rollout, you can’t target specific users, device types, OS versions, etc.
When you halt and then resume the rollout of your release, you’ll be affecting the same set of users.
When you do a staged rollout of a new release before completing the rollout of the previous release, the new release will use the same group of users as the previous release (depending on the percentage of the rollout).
Your app update will be available to the percentage of users in your staged rollout, but it may take time for the full group to receive the update.
Users won’t be notified if they receive a version of your app in a staged rollout.