Computers have a limited number of PCI slots; usually three or six. An expansion chassis is an external box that has a number of additional PCI slots (up to thirteen). When the expansion chassis is properly connected to the computer, the PCI slots of the chassis become an "expansion" of the computer, thereby allowing you to use more PCI cards than was originally possible. If the total number of PCI cards you need to install on your computer is greater than the number of available PCI slots on the computer, then you need an expansion chassis.