Forecasting is a step up from Modelling. Forecasting will give you a lot more control to manage a product, so we recommend you create forecasts for your most important inventory items. 
While AIM Modelling will calculate a rate of demand based on a past time period, Forecasting will allow you to set future demand expectations. Because of this, you can't use new products in Modelling, as without a history, the calculations come to zero. In forecasting, you can create a forecast for growth of a new product and plan replenishments. 
AIM Forecasting also has a much more detailed stock control system. While Modelling will use the average amount of demand to calculate stock limits, AIM Forecasting will calculate replenishment dates and sizes based on the amount of demand that occurs on the following days. This makes forecasting much more suitable for seasonal products, or managing a lifecycle, such as new products or end-of-life products.