Documents can be grouped in the storefront by adding a/multiple filter group category/categories to document entries in the OPDS feed.
<category scheme="http://schema.pugpig.com/filtergroup#date" term="2014" label="Date#2014 issues"/>
<category scheme="http://schema.pugpig.com/filtergroup#editiontype" term="special" label="Edition Types#Special Editions"/>
All document with the same filter group scheme and term will be grouped together in a filter. The fragment in the scheme represents the filter group, the term is the filter. You can use any valid fragment to group filters. We recommend using a semantically meaningful fragment like date, type, etc.
The label should match across all uses of a given scheme and term tuple. The label is of form "Group display name#Filter display name". (The group display name is for future use.)
Filters appear in alphabetical order by filter group name then by term value.
An edition can appear in multiple groups simply by adding as many filter group categories as appropriate.