The way it works is similar to Hitachi’s (now shut down) Gazopa: if a user needs to find another item that fits the one they are about to buy, FashionNavi finds those items, based on the color and the shape of the item selected.
For example, after I clicked on the jacket on the top left on the screen you can see below:
FashionNavi offers up this page, obviously with more items that might fit the jacket:
It’s also possible to use FashionNavi when you like a specific item but want to see it in a different color (choose the box on the top left).
FashionNavi is easy to understand even if you understand Japanese by the way: link.