The definition of Display fonts is quite loose. Generally speaking, display fonts are made to grab attention by incorporating some more extravagant visual features (think something like Papyrus)

They are made for shorter texts that are often written in a bigger font. Again I talk about this in a very general way because it depends on the font and other factors. But usually this includes things like headings. So they would use slightly different proportions that wouldn't work that well at small sizes, but stand out more in bigger sizes compared to the "text" variant.

So in this case you would use Zed Text for all your larger text blocks and Zed Display for headings or maybe emphasized words. But to be honest, since they are pretty close visually, you can get away with using Zed Text for everything imho.