Facets are represented as atom:link element at the top of an acquisition feed:

By default, we consider that a facet is inactive. If a facet is active, an attribute opds:activeFacet is added to the link and set to "true".

Facets can be grouped together using an opds:facetGroup attribute. In a group, only a single facet can be active at a time.

A facet can also advertise the number of entries expected using an element from the Atom Threading extension: thr:count

The so-called application stores (AppStore & Android Market), do a very poor job when it comes to discovery, and part of this problem is due to the lack of faceted browsing.

While facets are not as common on mobile applications as they are on websites (due to the limited screen space or the fact that designing mobile applications is still a new area of expertise) , a few design patterns already exist.

The following article at UXmatters is an excellent introduction to these patterns: http://www.uxmatters.com/mt/archives/2010/04/design-patterns-for-mobile-faceted-search-part-i.php

On larger devices such as tablets, specialized UI elements such as the iOS split view (Official guidelines) or Android fragments (Official documentation) are a perfect fit for facets.

On desktop and in web apps, the most common design for facets is to display them in a left-hand panel, but some websites also put them on top or on the right.

While there are no official recommendations in terms of UI from the spec or the OPDS group, the best way to treat facets is to display them where people would expect them the most (based on the platform you're targeting).

Feedbooks has a live catalog with facets available at http://www.feedbooks.com/catalog

Bluefire Reader is the first OPDS client to support this feature (see screenshots below).

The Internet Archive and Inkstone Software should also be among the first to support faceted search & browsing.