A data staging area is a storage area between the data sources and a the data warehouse. This staging area is mainly used to quickly extract data from its data sources, minimizing the impact on the sources. It also often contains all the data cleansing, validation and transformations, as well as de-normalization, so that when the data is moved to the final data warehouse data area it is ready to be consumed by data consumers.

You can add a data staging area by adding a data area in the Prepare instance called DSA and another data area called DW. Bring data from the Ingest storage into the DSA data area, and make the necessary transformations. Once the transformation have been applied, bring the tables from the DSA data area to the DW data area.  

The usage of a data staging area is optional.  It is simply a way of keeping the final data area clean without any business logic on the tables inside it.