Compared to the physical world, one of the online world's benefits is that items can live in multiple locations. Because websites can classify products and other content along multiple dimensions, they help users navigate locally to related items and provide faceted winnowing of a large product space into manageable shortlists that can satisfy the user's main requirements.

This is all good, but polyhierarchy can easily become a crutch. Rather than spend time upfront to develop several intuitive and logical top-level categories, teams rush through this important process, creating numerous weak categories and listing products multiple times within them. The usability impact? Users spend too much time agonizing over top-level categories and then get confused when they see items showing up in multiple places ("are these the same thing?").

With too many classification options and too many structured dimensions, users are forced to think harder to move forward. The profusion of options also makes people question the information scent. This lack of confidence early in the site experience extends throughout their visit and can negatively impact the end result (by thwarting a purchase, for example).