ONIX is used for sharing metadata about accessible titles in the book supply chain and EDItEUR has two application notes about the use of ONIX for accessibility metadata, an introduction and a more advanced one. 

All the information about ONIX in general can be found at this page: 
https://www.editeur.org/93/Release-3.0-and-3.1-Downloads/


Accessibility metadata in ONIX (essentials) can be found at this link:
https://www.editeur.org/files/ONIX%203/APPNOTE%20Accessibility%20metadata%20in%20ONIX.pdf


Accessibility metadata in ONIX (advanced) can be found at this link:
https://www.editeur.org/files/ONIX%203/APPNOTE%20Accessibility%20metadata%20in%20ONIX%20(advanced).pdf


ONIX uses code lists to communicate a controlled vocabulary from machine to machine. All these code lists are visible on an online browser here:
https://ns.editeur.org/onix/en

The most important ones for accessibility are:

There are other code lists as well, and all of these are listed in the EDItEUR documentation.