Army rankings for Warhammer Fantasy

I'm told that a number of Fantasy tournies use a system that ranks the various armies and applies a handicap - eg if you use a top-ranked army like Daemons you get fewer points for kicking Wood Elves than if you use something less powerful like say Dwarfs.

What I'd like to try and find is an example of this in practice to see whether it can be applied to our club's league games. We have about a dozen active players but none of us do tournaments at the moment.

Can anyone point me in the right direction please?

Regards
TT