BLS is the federal government. Congress and the president have the entire data set produced by BLS. They also have state-level data and more. There are no statutes explicitly tied to U3.
I mean using it as what's commonly associated with unemployment. There aren't any requirements to use U3, but the DOL uses it instead of U6. I think most people's idea of unemployment is closer to other measures than it is to U3.
U6 includes underemployment. It's definitely not what most people think of as unemployment. It doesn't adjust for participation rate. Participation can be misleading because while there are definitely discouraged workers the size of the workforce varies naturally as people retire, become disabled or take time off for families. U3 helps track demand for employment and anyone outside the labor force isn't demanding work.
That being said, the closest single metric to follow isn't any of the indices. Try prime age employment-population ratio.
