9.6 The Slug: Header
Slug is a HTTP entity-header whose value is a “slug” – a short name that can be used as part of the URI for a Member Resource.
When posting an entity to a Collection to add a new Member, the server MAY use this information when creating the Member URI of the newly-created resource, for instance by using some or all of the words in the last URI segment. It MAY also use it when creating the atom:id or as the title of a Media Link Entry (see Section 9.5.).
Servers MAY ignore the Slug entity-header and MAY alter its value before using it. For example, the server MAY filter out some characters or replace accented letters with non-accented ones, spaces with underscores, etc.
What a terrible idea.
I will be recommending internally that we never, ever use this header.