The engagement metrics are based on each cart + stage combo, but not unique people.
Example scenarios:
If a customer clicks on two different email stages that belong to the same cart it counts as 2 clicks.
If a customer clicks the same email twice it only counts as 1 click.
If a customer clicks on two different emails that belong to different carts (they created a cart, abandoned, then created another cart and abandoned), it counts as 2 clicks.