tiers element represents a pricing tier in your product.
The tier model
Use it to define each pricing tier individually. If your product doesn’t have tiers, it’s a single-tier product. Setup_to to NULL.
Use unit_amount when you charge per unit. For example, “User based pricing”, $10/user.
Use flat_amount when you charge one total cost for a given subscription, for example, “Premium plan”, that has prices for 200/year.