With API you can:
Get user's subscription status.
Activate subscription for a user with paid access level.
Get user's attributes.
Set user's attributes.
You can't get subscription events via API, but you can use Webhook or direct integrations with a service that you're using.
To correctly work with API you need to use a unique ID for your users. This may be an email, a phone number, your internal ID. Without such ID it's impossible to identify the same user on multiple platforms.
Whenever Web payment providers you use such as Stripe, ChargeBee, or any other, you can sync subscribers. For that:
Use a unique ID for your users. For example, email or phone number.
Check subscription status via API.
If a user is freemium, show him a paywall on Web.
After successful payment, update subscription status in Adapty via API.
Your subscribers will be automatically in sync with mobile.
Due to security reasons, you can't grant subscription via mobile SDK.
Imagine a case, when you run a promotional campaign with offers 7 days of a trial and you want to sync in with mobile experience. To do that:
Get a unique ID for a user.
Set premium access via paid access level with API with a duration of 7 days.
After 7 days users who won't subscribe will be downgraded to free tier.
You may have custom attributes for your users, other than defaults such as IDFA, device model, etc. For example, in a language learning service, you may want to save a number of words a student has learned. To do that:
Get a unique ID for a user.
Update attribute with API or SDK.
With such attributes you can, for example, create a Segment and run an A/B test or a Promo campaign.
To learn more about S2S API go to API Specs.