You can add an expansion unit to many of the iSeries™ servers to support additional features and devices.
If you want to create logical partitions on your iSeries server, you will probably need to add an expansion unit. This will contain the additional hardware that you need for each logical partition.
There are different types of expansion units. Some expansion units can support only disk units (storage expansion unit), while others can support a variety of hardware (system expansion unit). This is dependent on the kind of buses and IOPs that are installed in the unit.
Expansion units generally contain one or two system I/O bus with multiple IOPs that control various I/O devices.