Inventory Planning is part of SC Navigator. Its purpose is to calculate inventory levels that are required for your desired service levels and to give you insight in your actual stock status.
You can use the output of Network Design as input for Inventory Planning to see the effect of your designed network on your inventory levels.
You can use the output of Demand Forecasting as input for Inventory Planning to plan your safety stock levels.
You can use Inventory Planning results as input for S&OP Navigator.