Sign up on(Subscription Created): This is the date when the subscription was created. For API users, if you're listening to our webhooks, you will receive a subscription_created event every time a subscription is created.
Started on('Future' subscription starts) : If a subscription is created by specifying the start date, it would be created in "Future" state. The start date is when the subscription starts from Future state to Active state. If a subscription is created without specifying a start date, this date would be the same as signup date. If you're listening to our webhooks, for subscriptions created by specifying a start date, you will receive a subscription_started event when the subscription starts.
Activated On: This is when the subscription has been moved from "In Trial" to "Active" state. If you're listening to our webhooks, you will receive a subscription_activated event when a subscription is activated.
Here’s an example scenario for a subscription in different status and how the dates would vary.
1st March(Future status)
10th March(moves to active)
1st March(Trial status - 10 days)
10th March(Moves to Active)
Refer to this link for the complete list of events and when they will be triggered.