Not so fast. Unless I’m misreading their GIS website, the Navajo Nation has adopted the Census Bureau’s CDP boundaries as their own chapter boundaries (the third-order boundaries in the Navajo system). The data may come from the Census Bureau, but that’s the source of truth for these tribal administrative boundaries.
I don’t have a photo of a chapter entrance sign to share, but the chapters are used for geographical reference. For example, Lake Powell Navajo Tribal Park is described as being “located within the Le Chee Chapter of the Navajo Nation.”
@pnorman wants to avoid numeric levels. There may be some wisdom in this. It took us years to come to the realization that there are subdivisions beneath reservations, so what’s to say we aren’t overlooking some differences in the number of levels in each reservation that would frustrate any attempt at national consistency? On the other hand, the chapter boundaries are, practically speaking, spatially equivalent to local municipal boundaries, so maybe something should reflect that. And without some explicit scale, it’s going to be up to renderers to figure out how to make each increasingly local subdivision consistently show up only at higher zoom levels.