extensions and expected values

Hello folks,

A couple of questions which I hope aren’t too naive, but between reading this list and what documentation exists, it’s not always clear what best practices are for schema.org.

First, should we (the accessibility metadata group) be recommending that people use the “/” extension mechanism for accessible book formats like EPUB and DAISY. The extension page notes “http://schema.org/EBook/KindleFormat” as an extension of the bookFormatType enumeration, but is that URI syntax expected to last? Should we instead recommend using a more basic string like “DAISY3” or “EPUB3” to avoid future problems, or is the use of string values with bookFormat problematic in itself? Recent discussions on this list have cast some doubts.

And that leads to the other question we have, which is what to do when a needed data type doesn’t have an exact match in schema.org? If you have adapted a work to make it accessible and want to note that it is an adaptation of another, should we indicate an expected value of URL even though a URI is wanted, since URNs may be the only usable identifiers? In other words, is usage context more important than the expected value? For example, if I use link/@href for URLs and meta/@content for URNs, does it matter that the expected value is URL because it’s expected that most adaptations will have a referenceable source on a publisher’s site?

Thanks in advance for any insights that can be provided,

Matt