The Inventory Performance Index (IPI) is a score Amazon assesses to determine how well sellers are managing their inventory. You can ipi score amazon between 0 and 1000.

As with many of Amazon's decisions for sellers, the IPI seems to largely revolve around Amazon's overflowing warehouses and trying to get sellers to better optimize their inventory.

There are three factors that affect your IPI score:

Reducing excess inventory to increase profitability
Increasing sell-through to balance your inventory weeks of cover
Ensuring inventory is buyable by fixing listings that are stranded