Definition of 

Grace Period from Opt-In

What's 
Grace Period from Opt-In
: it is one of the possible values for
Event
. It means:
The App Store was unable to complete the transaction of renewing a subscription with a marketing opt-in bonus period due to a billing issue and the subscription enters a 6 or 16 day Billing Grace Period window.

See other fields from

Trends: Subscriptions Events

Explore more Reports from

App Store Connect

The (only) data hub made for apps