OS/400 Server and Feature Code - This server number refers to the model number of your system (i.e., E4D, 41A, 820, etc), while the feature code refers to the processor feature the model contains (i.e, EPXK, EPXD, 2248, etc). You can find this information from the green screen or in iSeries Operations Navigator. To find it on the green screen, enter the Start Service Tools (STRSST) command and select option 1 (Start a Service Tool) followed by option 7 (Hardware Service Manager) from the SST menu. On the Hardware Service Manager screen that appears, select F6 to print your configuration and the server number and feature code will be listed at the top of each page (you can also spot some other nice pieces of system information on this report, including the system serial number and your system type).
Another way to do this without using SST is to type WRKHDWRSC *PRC on the command line. The top line will be your machine type and model number. Type option 7 by the machine type and the processor feature code will be displayed, along with the serial number.
It's even easier to find this information in Ops Nav. Under your AS/400 icon, open the Configuration and Service->System Values->System Control node. Click on the System tab of the System Control panel that appears, and you'll find the processor group and serial number listed on this screen.