As of 4.6A, PC documents and binary documents are stored in the Business Workplace internally by the SAP Knowledge Provider. You can reduce the load on the R/3 database by storing the content of these documents on an external content server. A further advantage is that the documents stored on the external content server no longer have to be accessed via an application server when the SAP MAPI Service Provider is used.
For further information, see the documentation on BC - Business Workplace.