https://learn.microsoft.com/en-us/exchange/client-developer/web-service-reference/subscriptionid-getstreamingevents
SubscriptionId (GetStreamingEvents) | Microsoft Learn
The SubscriptionId element represents the identifier for a streaming subscription.
microsoftlearn
https://developer.mozilla.org/en-US/docs/Web/API/PushSubscription/subscriptionId
PushSubscription: subscriptionId property - Web APIs | MDN
The subscriptionId read-only property of the PushSubscription interface returns a string containing the subscription ID associated with the push subscription.
web apispropertymdn