System disk is used to cold boot a PC.
It is the system disk because the other disks may boot a PC, but they also perform added functions beyond booting the PC. To perform a cold boot (also called a "hard boot") means to start up a computer that is turned off. It is often used in contrast to a warm boot, which refers to restarting a computer once it has been turned on. A cold boot is typically performed by pressing the power button on the computer.
A system is a regularly interacting or interdependent group of units forming an integrated whole. Every system is delineated by its spatial and temporal boundaries, surrounded and influenced by its environment, described by its structure and purpose and expressed in its functioning.
A system boot disk is a floppy disk which allows your computer to boot into an operating system such as DOS. Examples include the DOS installation disk and the Windows 95/98/Me startup disk. You can use a startup or system disk to clean boot a Windows 95/98/Me computer.