The end of the section on Indian reservations on the United States admin level page on the wiki is very vague

[stevea]

I also offer support for Minh’s “floated” idea. We’ve gotten a lot of excellent, much-more-clear work done with tagging distinctions apart from admin_level=* using boundary=* tagging. Similarly, boundary=indigenous_administrationis a logical extension to this key which can push ahead such syntactic growth into a semantic space where it can offer similar distinctive clarity. Tag well!