There are a lot of metadata boxes in current use for audio and video files that apparently have never been registered, or are at least missing not documented at the MP4RA site (and in this repository). Many of them originate in Apple iTunes (or Quicktime), but are supported by other software as well, based upon reverse-engineered documentation I suppose. Some of these start with the copyright symbol, e.g. ©ART or ©nam, to mark them as private-use obviously, but others are not, e.g. stik, hdvd, tves or purl.

The following points are not clear to me:

1. Should all of these be registered, or just ones not starting with $a9?
2. Would registering them be possible at all without a formal and publicly available specification?
3. If a public specification is needed, but the vendor is not willing to release one, would it be acceptable to create and publish one elsewhere?
4. If so, would this require backing from a formal standardization body or a company, or could it be done as a collaborative open-source effort?