What does the boot system command do on a Cisco switch, and what happens if the BOOT variable is not set?

Sets the BOOT environment variable that tells the switch which IOS image to load from flash; if it is not set, the switch just loads the first executable file it can find.

* Setting BOOT makes the running image a decision rather than an accident. *

S1(config)# boot system flash:/c2960-lanbasek9-mz.150-2.SE/c2960-lanbasek9-mz.150-2.SE.bin

Command breakdown:

If the BOOT variable is not set: the switch loads and executes the first executable file it can find in flash. That is exactly why pinning the image matters. A switch that has collected two IOS images — after a half-finished upgrade, say — would otherwise boot whichever one the file system happens to hand over first, so the version running on your network becomes an accident rather than a decision. Setting boot system makes the choice explicit and survives the next reload.

Once IOS is up it applies the commands in the startup-config file, which on a switch is named config.text and also lives in flash (a router keeps its startup-config in NVRAM (Non-Volatile Random-Access Memory) instead). Read the current setting back with show boot.