"All But One" principle

[MN BB Ref]

Hi,

Still being a newbie football ref. I'm not up on everything yet. I've been doing my homework but there is still one thing I haven't figured out yet, and that is what exactly is the "All but one" principle. I seen it referred to on our exam this year but I haven't been able to ascertain exactly when this is applied or what it means.

Can you vets help me out here?

Thanks!