Combined warehouses provide a "global" view of your sales and stock information, allowing you to generate purchase order recommendations based on total available stock against available against total relevant sales.

By default, a combined warehouse calculates its own independent sales velocity, forecast, and replenishment recommendation based on the sales and stock information feeding in to it, so it will be a separate result from summing the connected warehouses.

This feature is useful for situations where you can transfer stock between warehouses, provided stockouts aren't an issue.

The "Combine forecast" option in the combined warehouse settings is helpful when stockouts are an issue in one particular warehouse (e.g. FBA), resulting in a higher forecast being generated as opposed to the forecast that would otherwise be generated based on the combined stock.

The "Combine forecast" option considers the forecasted sales of each child warehouse (instead of creating an independent forecast) against the combined stock, then applies the lead time and days of stock input in the combined warehouse view to give replenishment recommendations.

Past stockouts can have an impact on the calculation of sales velocity when the "Use stockouts history" setting is enabled. If you are out of stock frequently in one location, the sales velocity and forecast will increase accordingly for that location. The assumption with a Combined Warehouse is that stock can be shared/transferred between locations, so if there is any positive stock it will not be considered a stockout day. Override the past stockouts in your Combined Warehouse if needed directly in Inventory Planner, or using a CSV import.

More information: What are stockouts? How do stockouts affect the forecast?

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.