The Ethnicity and Race Domains are both determining the ethnic and racial background of a person. It follows the recommendation of the Executive Office of the President, Office of Management and Budget (OMB), Office of Information and Regulatory Affairs of the US Administration (the White House). This classification provides a minimum standard for maintaining, collecting, and presenting data on race and ethnicity for all Federal reporting purposes.
The way it works is this: The race field contains races and ethnic backgrounds, while for Ethnicity there are only two categories for data on ethnicity: “Hispanic or Latino” (concept_id=38003563) and “Not Hispanic or Latino” (concept_id=38003564). This means, the two categories are orthogonal to each other, and both Latinos and non-Latinos can have any racial or ethnic background.
This is a very US-centric solution, and hence the terminology might be confusing to non-US data owners. If belong to the latter group, you can probably ignore this field entirely.
There are no relationships defined for Ethnicity.