U1, persons unemployed 15 weeks or longer, as a percent of the civilian labor force;
U2, job losers and persons who completed temporary jobs, as a percent of the civilian labor force;
U3, total unemployed, as a percent of the civilian labor force (this is the definition used for the official unemployment rate);
U4, total unemployed plus discouraged workers, as a percent of the civilian labor force plus discouraged workers;
U5, total unemployed, plus discouraged workers, plus all other marginally attached workers, as a percent of the civilian labor force plus all marginally attached workers; and
U6, total unemployed, plus all marginally attached workers, plus total employed part time for economic reasons, as a percent of the civilian labor force plus all marginally attached workers.
discouraged is generally measured as short term for the last 4 weeks actively looking for employment, and long term as within the last 1 year
anyone who is actively looking for work past the 1 year mark aren't even considered, which affects people of 2 varieties, a skilled job that has been wiped out in the US, or a low skill job wiped out of a region. both of these people still want jobs, but have taken quite a bit of time searching: still technically unemployed
honestly if you consider the way industrial production and manufacturing in the US was on the downshift, plus many construction jobs, it's fairly obvious U6 isn't necessarily the hands down best.
(i'm addressing its aggregate with U4 specifically btw)