The base 603 is a processor socket for Intel server processors of the Xeon series. It was used by Intel for workstation and server platforms.
Intel differentiates the base 603 from the base 604 as a "low cost, low risk, robust, high volume manufacturable, and multi-source -fashioned". Therefore all socket 603 processors can also be used in motherboards with socket 604. Conversely, this is not possible.