The Core chapter, section 3.3.3, contains this:
Like other bibliographic references, the citation attached to a quotation may be represented simply by a pointer, as in this example:
Lexicography has shown little sign of being affected by the
work of followers of J.R. Firth, probably best summarized
in his slogan, <cit>
<quote>You shall know a word by the company it keeps.</quote>
The introductory text says that the example will contain a pointer, but it doesn't. It's not really clear whether the intent was to use a <ptr> tag, but either the text or the example needs to be changed.