The referrer property appears in the current (11 February 2016) version of the EPUB for Education Structural Semantics document http://www.idpf.org/epub/profiles/edu/structure/structure-20160211.html . That document states
"The semantic terms defined in this document are all included in in the EPUB Structural Semantics Vocabulary [StructureVocab] and available for use in the epub:type attribute [ContentDocs301]."
In addition, EPUB books containing the attribute
can pass as valid EPUB 3.0.1 documents through the current version of EpubCheck, 4.0.1 . So it might seem that the referrer property is safe to use in epub:type attributes in EPUB 3.0.1.
But the referrer attribute is actually *not* in EPUB Structural Semantics Vocabulary https://idpf.github.io/epub-vocabs/structure/ . So, since the EDUPUB specifications are apparently still in public draft status http://idpf.org/edupub , maybe the promise in the EPUB for Education Structural Semantics document should for now be interpreted only as an indication that all of the semantic terms in the EPUB for Education Structural Semantics document *will* appear in the EPUB Structural Semantics Vocabulary when (and if) the former becomes a finalised standard (unless the promise is removed by then), not as a promise that those semantic terms are all there already. But then there's EpubCheck, which generally rejects EPUB books with epub:type attribute values that contain semantic terms not in the default vocabulary, but accepts books containing epub:type="referrer" .
So it seems that either the EPUB Structural Semantics Vocabulary is wrong and referrer should already be in there, or EpubCheck 4.0.1 has a bug and should refuse books containing epub:type="referrer" attributes, or maybe both (in that referrer should be promptly added to the Structural Semantics Vocabulary but EpubCheck should fail books that contain it until it is added). Which of these is the case, or is there something else I am missing?