Some SAP applications such as the Business Document Service normally store their contents in the SAP database. However, you can also use external content servers such as the HTTP Content Server to store contents. After installing the necessary hardware, you can change the default category for your document class so that the external content server is used for storage.
You must have maintained the category (see Maintaining the Content Category). You can then change the default category (see Changing the Default Category).
You can find more information in the SAP Library under Basis - Basis Services - SAP Knowledge Provider.