Pivotal Events are game changers in the Event Storming sessions. They can be used as:

• a sorting strategy
• candidates for system's boundaries
• markers for the transition between different business phases
• a helper to look for places where exchanging business value for some other value (for example money, reputation) happens

Pivotal events change the rules of the game. They're highlighting important things happening in our domain.
For example, for an e-commerce website it could be Article Added to Catalogue, Order Placed, Order Shipped, Payment Received, and Order Delivered. These are often the events with the highest number of people interested. Also, those can trigger reactions in other departments.
Once a pivotal event happens, the flow might be a little bit different. Let's look at a very simple example.

Once the order is placed, in most places can be changed, for any reason. But once the order is shipped... Well, the change in the order is much harder to do. That's the pivotal event. Once it happened, the rules of the game changed. The time for changing the order has passed. We may need the policy to handle those cases.