Great explanation by dsmawd.
Also remember, if a tire only has a "Z" rating without clarification whether it's a "W" or a "Y", it means it has to be designed a few years back at least. Since most tire manufacturers update their tires with better design and material often, I'd NOT get such a "Z" rated tire. Such rating really means nothing other than -
"It's better than a 'V'-rated one from a few years back".