Final answer:
A switch uses the BOOT environment variable to determine the location of the IOS image it needs to load during startup.
Step-by-step explanation:
A switch uses the BOOT environment variable to locate and load the IOS image. When a Cisco switch boots up, it checks the BOOT environment variable to determine the location of the IOS image file it needs to load. If the BOOT environment variable is not set, the switch will attempt to load the first IOS image it finds in flash memory by default. The IOS image is the operating system used by Cisco devices to provide network services.