Boot system commands are just another tool you can use to manipulate your router. Using this command, you can configure your router to boot from a different IOS in flash, or have it boot from a TFTP server, even though that is not practised anymore; TFTP servers are mainly used for back-up purposes.
So, let's look what we can do with the boot command and all of its sub-commands:
Since this is a simulator, when you type the question mark to see what sub-commands are available underneath the command boot, you only get the system sub-command since this is what the certification calls for. Also, when you type the command boot system with the question, you get some more options such as word, flash, and TFTP. These sub-commands are specific to the certification, and there a lot more options than you see here, but again you could always question mark your way...
