That was defined and settled years ago. An ADOS/FBA/African-American is defined as: a person who has identified as Black or African-American on at least 2 censuses and has ancestry of the Descendants of Enslaved Africans in the USA going back to 1870 (which would have been the 1st census after the Civil War).
If someone else has a clearer/better/more official definition, please do correct.