ZYX cartridges

Can someone explain to me the heirarchy of ZYX cartridges? I guess I know that the Universe is top of the line (though the Atmos may be even more expensive). But beyond that there seems to be umpteen different variations of the Airy model, called "2", "3", etc, with silver vs copper windings, etc. Since these are said to work well with my Triplanar tonearm, I am thinking of making a purchase, but I am completely confused regarding their product line. Thanks.