Timeline sets (or collection sets) allow you to deliver different content to different users, based solely on user choice.
You can assign certain timelines or editions to a set, and if so only users who have chosen that set, will see that timeline in the app. Users make this choice at first launch on a screen that shows before or amongst the original app onboarding screens, but it can also be included as an option in settings.
You can assign more than one set to a timeline, and any timeline or edition not specifically assigned to a set will be visible to all users.
A common use case for using sets is when you have readers in different regions. New Statesman, for example, have a UK edition and an International edition. They deliver both sets to the app, but users will only see the one they opt in to.
Sets could also be used for language localisation, or any other way you may have to split your content.
NB - Timeline sets have no connection to your app's authentication. All users have the ability to see all sets regardless of their access.