Subscriptions for a given period are the number of transactions that start an active subscription. While Subscribers are just a sum of all users with an active subscription for a given period.
Users can opt-out of subscription renew (turn off auto-renew). Technically they are still subscribers, but they won't pay you in the next period. It's important to know how many users won't prolong a subscription as it affects your payroll for the next month.
The easiest way to do so is via App Store Connect / Google Play Console. However, you won't have detailed analytics and control over your data.
So the best way to do so is to handle Apple/Google server2server notifications and build custom views.
Adapty provides you with:
You get all this data for iOS and Android instant as soon as you install SDK.