IBM Racks come in a number of different types basically speaking.
For xSeries, there are several types of racks :
- NetBAY42 Standard Rack (19")
- NetBAY42 Standard Expansion Rack (19")
- NetBAY42 Enterprise Rack (19")
- NetBAY42 Enterprise Expansion Rack (19")
Basically, if you want a normal 42U rack for your xSeries, the you go for the Standard rack. The Standard Expansion is for additional racks to keep your servers. These expansion racks do not have sidepanels. Usually, you will purchase 2xStandard Racks to hem in the other expansion racks.
The same for the Enterprise Rack, but these racks are far sturdier than the normal ones, and can hold heavier items than the standard ones, and they are wider. Basically these enterprise racks are almost the same as the pSeries ones, except that pSeries racks are better quality and has some additional features.
Cheers.