In the Item Detail view, scroll down to the history table. The history table will contain the sources of demand. If a customer(s) is listed in the history table, simply click on the customer name. The customer name is hyperlinked to the demand plan for that specific customer, item, and location relationship. In the image below, we will click on Salmanca.

In the Demand Plan view, a "Temp" page will be created that shows the context of the location > item > customer

You can then click on the level above to see the total demand plan of the item in the specific location across all customers. In the view below, the Demand Planning Stacked Grid Editing has been selected in the Statistics grid to see all the customers together.