The "Opt In" event ($opt_in) is triggered when the opt_in_tracking() method is called on the Mixpanel SDK. [Protecting User Data]
Specifically, this happens when:
A user opts into tracking after having previously opted out, or
A user opts in when the SDK was initialized with tracking opted out by default (via opt_out_tracking_by_default).
When the Opt In method is called, it sends a $opt_in event (displayed as "Opt In" in your Mixpanel project) to indicate the user has consented to data tracking, and sets a flag in the user's cookie/local storage to enable tracking going forward. [Protecting User Data]
Note: The $opt_in event is a non-qualifying event and is excluded from both event-based and MTU billing calculations. [How Mixpanel Pricing Works; MTU billing]
