The combined warehouse calculates a separate forecast based on data connected to it (sales and stock information), so it will be a separate result from summing the connected warehouses.
This means that a new sales velocity is calculated, which means there will be a different forecasted demand for all variants.
Lead time and days of stock can also be set for each warehouse. That means that replenishment recommendations are specific to lead time and days of stock settings for the combined warehouse.
When creating a purchase order using a combined warehouse for forecasting and replenishment calculation purposes, you will need to select a different warehouse as the destination warehouse. The combined warehouse is a 'virtual warehouse' whereas the destination warehouse specifies a physical warehouse where inventory will be received.