emergency repair disk – what value?

Every time I read about the emergency repair disk for NT4.0, it leaves me with a question. If you can only put so much on the repair disk, how can it work properly? I was thinking that it must pull information off the hard drive you are attemptingto repair, but I don’t really understand. In order to truly repair it, I would think it would need much more information than it can store on a floppy disk. Can someone clarify how it actually works?