It’s an overly fancy word, but we are already using it intuitively on most e-commerce websites and library catalogs.
Here is an article how Polyhierarchies Improve Findability for Ambiguous IA Categories , the Nintendo Switch is part of both the “electronics” and the “video games” category.
The Amazon example on the Knowledge Management for Tags proposal shows how faceted searches can be based on this.
Breadcrumbs can show one or multiple hierarchies at the same time.