If the product you are forecasting is an assembled item, then you will see the Plan Production panel. 

Would you like to link components?

Turning this option to 'Yes' will prime the production planning system to calculate the required replenishment in this forecast and apply that as demand to all the components of this product in the Bill of Material. This calculation is triggered when you select 'Save Forecast' in the top right of the forecast page. 

After saving your forecast you will see notification that the production demand is being calculated:

Once the process is complete you will see a green status icon appear:

Source Warehouse

Because you may have the components stored in a different location than where the assembled items are kept, you are able to select the source warehouse location. Selecting the source warehouse where the components are kept will mean the production demand will be added to the forecast of the components in the source warehouse. This in turn will trigger replenishments for the components in the source warehouse.