(tested with EPUB Check 4.2.2 - via the Pagina Mac app)

http://kb.daisy.org/publishing/docs/metadata/evaluation.html#a11y:certifierCredential

The property can either be a text field with the name of the credential held, or it can be a link to a description of the credential.

Relevant test files:

https://github.com/w3c/epubcheck/blob/master/src/test/resources/30/single/opf/valid/vocab-a11y-declared.opf#L11

https://github.com/w3c/epubcheck/blob/master/src/test/resources/30/single/opf/valid/vocab-a11y-undeclared.opf#L10