A Subscription Group is a concept developed by Apple to host a set of related In-App Subscriptions and define the up, down and cross-grade logic within your product portfolio.
It also ensures that a user can only subscribe to, and be charged for, one of the subscription plans belonging to the same Subscription Group.
Typically a Subscription Group is used in the following cases:
A tier variation of your product or offer (e.g. Bronze, Silver, Gold) giving access to more content or feature;
A duration variation of accessing your product or offer (e.g. Weekly, Monthly, Yearly)
When an existing subscribed user switches to another plan within the same Subscription Group, the App Store automatically applies a migration policy which varies depending on the nature of the change (up, down or cross-grade).
This logic needs to be informed by ordering subscriptions inside the Subscription Group in App Store Connect. This order defines the "level" of the subscription and the associated migration policy.
When going from a lower to a higher value (e.g. from Monthly to Yearly, in this example), the App Store will apply the UPGRADE policy. The upgraded subscription is immediately cancelled with a prorated refund, and the new one is immediately activated.
When going down from a higher to a lower value (e.g. from Monthly to Weekly), the App Store will apply the DOWNGRADE policy. The downgraded subscription will run until the end of its current billing cycle and the new one will automatically start.
When switching between subscriptions with the same level and number, the App Store will apply the CROSSGRADE policy.
The logic here depends on the subscription duration. If the duration is the same, the change is immediate. If they are different, the new subscription goes into effect at the next renewal date.
More information on this can be found here.
What about the other stores?
Google Play Store
Subscription groups exist on Google Play Store as Subscriptions.
The Purchasely platform will inform the store which migration policy to apply, depending on the Products & Plans configuration set in the Purchasely Console. For simplicity's sake, the same migration policies as on the App Store have been implemented.
Subscription groups can be defined in the HMS Console but the migration policies are slightly different.
Amazon App store
Subscriptions groups are named Subscriptions.
What about the Purchasely Console?
In the Purchasely Console, we have set a hierarchy of Products and Plans, to ensure migration policy can be enabled.
The Subscription Group thus becomes a Product in the Purchasely console where you can group Plans together.
Plans can be ordered inside a Product the same way subscriptions are in a Subscription Group.
Make sure your Plans hierarchy mapping matches between the Stores and the Purchasely Console. (see this page for further details)
In case your set-up is inconsistent between the stores, App Store rules have priority over the Purchasely Console.