Capitalization of "tribe" and its derivatives in the United States

It appears (from a comment) that you're asking about the US Government Publishing Office style guidelines.

It appears that according to the 2016 edition of the GPO Style Manual, tribe and tribal should always be capitalized when they refer to federally recognized American Indian tribes.

So it would be: "Many members of the Suquamnish Tribe live on the Port Madison Reservation. The Tribal leaders work on projects that provide cultural, educational, and recreational resources for the community. The Tribe has 890 enrolled members."

But: "The survivors of the shipwreck formed two tribes."

And: "The tribal delegation from Greenland arrived yesterday."

Names of organized bodies
3.18.

...

American Indian and Alaska Native federally recognized entities:
Shawnee Tribe, the Tribe; Cherokee Nation, the Nation; Alturas Indian Rancheria, the Rancheria; Cahuilla Band of Indians, the Band; Takotna Village, the Village; Akiak Native Community, the Community

(emphasis added)

4. Capitalization Examples

...

Tribe; Tribal (federally recognized)

This is somewhat counter-intuitive, both because it's different from many other guidelines and because it doesn't seem to fit a consistent pattern within the GPO guidelines.

For example, we're told to always capitalize Navy, but naval is not capitalized. You'd think that the same rule might apply to Tribe and tribal.