The Browser Accessibility Tree | The Paciello Group – Your Accessibility Partner (WCAG 2.0/508 audits, VPAT, usability and accessible user experience)

The accessibility tree is a subset of the DOM tree. It includes the user interface objects of the user agent and the objects of the document. Accessible objects are created in the accessibility tree for every DOM element that should be exposed to an assistive technology.
https://www.paciellogroup.com/blog/2015/01/the-browser-accessibility-tree/