It is not really part of any of the major subsystems as I have defined them, but it interacts with all of them. It is usually specified and purchased at the same time as the power supply, but the power supply is a separate component. The most popular form factor today is ATX , including its variants such as mini-ATX and microATX . A case that can accept more than one form factor is of course more flexible and will support both older and newer motherboards and power supplies. On a new system the case is usually chosen to match the motherboard, but on an upgrade the motherboard may be chosen to fit the existing case.
Size is also an issue in some situations.
While the form factor standard should be sufficient guarantee that a motherboard of the same form factor will fit in the case, it sometimes isn't. If you are planning to use a very large motherboard be sure it will fit in the case you are considering. Cases are available in either desktop or tower styles. Tower cases are usually available in a wider variety of sizes. This is largely a matter of personal choice, and matching to your furniture and space requirements.